Selectivity in Propene Polymerization with Metallocene Catalysts

Chem. Rev. 2000, 100, 1253−1345

1253

Selectivity in Propene Polymerization with Metallocene Catalysts Luigi Resconi,*,† Luigi Cavallo,‡ Anna Fait,† and Fabrizio Piemontesi† Montell Polyolefins, Centro Ricerche G. Natta, P.le G. Donegani 12, 44100 Ferrara, Italy, and Dipartimento di Chimica, Universita` di Napoli “Federico II”, Via Mezzocannone 4, 80134 Napoli, Italy Received September 16, 1999

Contents I. Introduction II. Basic Concepts A. Structure of Group 4 Metallocenes B. Activation C. Elements of Chirality D. Polymerization Mechanism E. Mechanisms of Stereocontrol in Primary Insertion (Site vs Chain-End Control) F. Regiochemistry of Propene Insertion G. 13C NMR Analysis of Polypropenes III. Elementary Steps A. Alkene-Free Species B. Agostic Interactions C. Olefin Coordination D. Insertion E. Insertion Barrier F. Chain Release G. Formation and Reactivation of Mt−Allyl Species H. Mechanism of Enantiomorphic Site Control in Primary Insertion I. Mechanism of Chain-End Control in Primary Insertion IV. Stereocontrol: Influence of the Catalyst A. Isotactic Polypropene: C2-Symmetric Metallocenes 1. Chiral ansa-C2-Symmetric Metallocenes 2. Unbridged Isospecific 3. Combining Two Symmetries: C2-meso-Cs B. Syndiotactic Polypropene: Cs-Symmetric Metallocenes C. C1-Symmetric Metallocenes: from Hemiisotactic to Isotactic Polypropene V. Stereocontrol: Influence of Polymerization Conditions A. Influence of Monomer Concentration 1. C2-Symmetric Catalysts 2. C1-Symmetric Catalysts B. Influence of Polymerization Temperature C. Epimerization of the Primary Growing Chain VI. Statistics of Polymerization A. General Remarks B. Mechanisms of Stereocontrol and Statistical Models 1. Chain-End Control (Bernoullian Model) 2. Enantiomorphic-Site Control

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IX. X. XI. XII.

C. Use of Matrix Multiplication Methods in Statistical Models Regiocontrol A. Stereochemistry of Regioirregular Insertion 1. 13C NMR Analysis 2. Molecular Modeling B. Reactivity of a Secondary Growing Chain 1. Copolymerization with Ethene 2. Chain Release C. Regioselectivity: Influence of the Catalyst Structure 1. Influence of the Metal 2. Influence of the Cocatalyst 3. Influence of the π-Ligands: Experimental Data 4. Influence of the π-Ligands: Molecular Modeling Analysis 5. Relationship between Regioselectivity and Type of Stereoselectivity D. Influence of Monomer Concentration E. Influence of Polymerization Temperature F. 2,1 f 3,1 Isomerization Mechanism Kinetics A. Activity versus Metal B. Activity versus Catalyst/Cocatalyst Ratio C. Activity versus Time D. Kinetic Models: Activity versus Monomer Concentration E. Activity versus Temperature F. Activity versus Solvent G. Molecular Weight Influence of Hydrogen Outlook Acknowledgments References

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I. Introduction The outstandingly rapid scientific and technological development of metallocene-based catalysts for olefin polymerization is a perfect example of the successful application of organometallic chemistry to homogeneous catalysis1 and of the teaching that understanding reactions at the molecular level can provide to the more matter-of-fact fields of heterogeneous ca* To whom all correspondence should be addressed. E-mail: [email protected]. † Centro Ricerche G. Natta. ‡ Universita ` di Napoli “Federico II”.

10.1021/cr9804691 CCC: $35.00 © 2000 American Chemical Society Published on Web 03/25/2000

1254 Chemical Reviews, 2000, Vol. 100, No. 4

Luigi Resconi was born in 1958 in Brescia, Italy, and obtained his Doctor in Chemistry degree from the University of Milano in 1984 with a thesis on Ni and Fe anionic carbonyl clusters under the direction of Giuliano Longoni. After a scholarship at the ETH-Zu¨rich with Piero Pino, where he begun to study C2-symmetric group 4 metallocenes, in May 1985 he joined the Ziegler−Natta research group at the Istituto G. Donegani (then the Corporate Research Center of Montedison) in Novara, Italy. Throughout 1989 he has been a visiting scientist at Stanford University in the group of Robert Waymouth, where he started the investigation on the cyclopolymerization of 1,5-hexadiene catalyzed by zirconocene/alumoxane systems. Since 1990 he has been project leader of the Homogeneous Catalysis research project, first with Himont, and then with Montell Polyolefins since its formation in 1995. He is currently senior scientist and program manager at the Centro Ricerche G. Natta of Montell in Ferrara, Italy, where he contributed to the development of several new metallocene catalysts for the synthesis of polyolefins, work that led to over 40 patent applications and several scientific publications. Current research activities of his group include the design and development of new homogeneous catalysts for olefin polymerization, the investigation of chain transfer and isomerization reactions in the stereospecific polymerization of 1-olefins, and the improvement of synthetic strategies aimed at more efficient preparations of metallocenes and other organometallic catalysts.

Resconi et al.

Anna Fait was born near Brescia, Italy, in 1961, and obtained her Doctor in Chemistry degree from the University of Milano in 1985 with a thesis on Ni polycarbido carbonyl clusters under the direction of Giuliano Longoni. After a brief experience in the organic chemistry department of Societa` Italiana Resine, she joined the Reactor Engineering Department of Montedipe in 1986, where she dealt with process optimization, later becoming leader of the same group. In 1993 she joined Himont (now Montell). She is now responsible for the Engineering and Research Development group of the Centro Ricerche G. Natta of Montell in Ferrara, Italy. Her group’s activities cover unit operations study, kinetic investigation, determination of chemical and physical parameters, and catalyst and process development.

Fabrizio Piemontesi was born in Borgosesia, Italy, in 1960. He obtained his Doctor in Chemistry degree from the University of Milano in 1987 with a thesis on alkylation and transamination reactions of amino acid residues coordinated on metal ions, under the direction of L. Casella. After a postdoc at the University of Milano with L. Garlaschelli, where he studied the synthesis of Ni and Pd phosphorane complexes, in March 1989 he joined the Ziegler−Natta research group of Himont at the Istituto G. Donegani in Novara, Italy, where he begun his activity on group 4 metallocene research. After the relocation of the group to the Centro Ricerche G. Natta of Montell in Ferrara, he shifted his research interests to the NMR analysis of polymers and polymerization statistics. Luigi Cavallo was born in Pozzuoli, near Naples, in 1962. He obtained his Ph.D. in Chemistry in 1990 under the direction of Paolo Corradini and Gaetano Guerra, with a thesis on the mechanisms of stereospecific polymerizations. He has been a visiting scientist at the University of Calgary in the group of Tom Ziegler, where he contributed to the development of combined quantum mechanics/molecular mechanics techniques. His research interests focus on studies on the mechanisms of 1-olefins polymerizationswith both homogeneous and heterogeneous catalystssand of conjugated dienes and styrene, and on the mechanisms of organometallic reactions, especially the enantioselective ones, as the (salen)− Mn catalyzed epoxidation of olefins. He is interested in the development of computational methodologies also.

talysis2 and material sciences.3 Indeed, titanocene dichloride4 was used in combination with aluminum alkyl chlorides as early as 1957 to provide soluble and chemically more defined and hence better un-

derstandable models of the TiCl3-based heterogeneous polymerization catalysts.5-10 However, the early catalysts based on Cp2MtX2/AlRCl2 or AlR3 (Cp ) cyclopentadienyl, Mt ) metal, R ) alkyl group) had a quite low activity in ethene polymerization and failed to homopolymerize 1-olefins altogether. Analogous research with zirconocene dichloride in combination with AlR3 was started by Breslow11 but met with limited success, until the serendipitous discovery of the activating effect of small amounts of water12 on the system Cp2MtX2/AlMe3 (X ) Cl or alkyl group)13 and the subsequent controlled synthesis of methylalumoxane (MAO) by the group of Sinn and Kaminsky14 provided organometallic and poly-

Propene Polymerization with Metallocene

mer chemists with a potent cocatalyst15 able to activate group 4 metallocenes (and a large number of other transition metal complexes, too) toward the polymerization of virtually any 1-olefins as well as several cyclic olefins.16 However, the activity of Cp2MtX2/MAO catalysts, although impressive toward the homo- and copolymerization of ethene, was moderate with propene and, more important, did not produce stereoregular polymers. Very low molecular weight, atactic oils were obtained in all cases. Besides the availability of suitable organometallic cocatalysts, the development of stereoselective, practical metallocene-based catalysts required the development of chiral, stereorigid metallocenes17-19 and of many new organic and organometallic reactions. Between 1984 and 1986, two key discoveries were made: the effect that different alkyl-substituted cyclopentadienyl ligands can induce on metallocene performances in olefin polymerization (the ligand effect)20,21 and the discovery that stereorigid, chiral metallocene catalysts can induce enantioselectivity in 1-olefin insertion.22,23 Since then, thanks to the combined efforts of industrial and academic research groups worldwide, an impressive leap forward toward the knowledge of, and control over, the mechanistic details of olefin insertion, chain growth, and chain release processes at the molecular level has been made. The success of group 4 metallocenes in olefin polymerization arises not only from their intrinsic understandability in terms of “simple” steric effects but also from the challenge they posed in terms of organic and organometallic syntheses: while the heterogeneous catalysts are much more difficult to study in terms of elementary steps than the homogeneous ones, the latter in turn are in general more difficult to synthesize. The synthesis of the ligands and the corresponding metallocenes have been reviewed.24-27 There is no unambiguous chemical definition for what a “metallocene catalyst” is. Obviously, not all biscyclopentadienyl transition metal complexes (metallocenes) are, or can be turned into, olefin polymerization catalysts. For the purpose of the present review, we limit the definition of metallocene catalysts to the biscyclopentadienyl complexes of transition metals of group 4 (titanium, zirconium, or hafnium) as well as a few examples of group 3 metals (Sc, Y, La). Therefore, the monocyclopentadienyl (e.g. CpTiCl3) and monocyclopentadienylamido complexes (e.g. the so-called “constrained geometry catalyst” Me2Si(Me4Cp)(N-t-Bu)TiCl2 and the like) are not discussed here: for recent reviews of homogeneous polymerization catalysts based on soluble, welldefined, nonmetallocene complexes of transition metals, see refs 28-30. Among all catalysts for the linear polymerization of hydrocarbyl olefins, the class of group 4 metallocene-based systems is the only one enabling control over the whole range of molecular weights (from olefin dimers and oligomers, to ultrahigh molecular weight polymers) and microstructures (stereoregularity, regioregularity, comonomer distribution) of polyolefins in a very wide range, making possible the

Chemical Reviews, 2000, Vol. 100, No. 4 1255

synthesis of improved and new polyolefin materials. The most important progresses in the field, which have largely outpaced those made in heterogeneous Ti-based and Cr-based catalysts, are the understanding of the catalyst structure/polymerization mechanism/polymer structure relationships and, in part as a consequence of it, the synthesis of a large number of novel polyolefin structures, such as random, stereoblock, and syndiotactic polyolefins. Because of the large capital investment required, a large part of research has been done in industrial laboratories, and some results have only been reported in the patent literature. Metallocenes are now successfully employed in the industrial or preindustrial production of several different ethene-based polymers, in different processes, from solution to slurry to gas-phase. The feasibility of adapting metallocene catalysts to the production of propene-based materials, such as isotactic and syndiotactic polypropenes, in existing processes has been demonstrated either in pilot or industrial plant scale. The polymerization of propene and higher 1-olefins introduces the problems of stereoselectivity (enantioface selectivity or enantioselectivity) and regioselectivity.31-33 Since enantioface selectivity requires stereorigidity in addition to proper ligand symmetry and metallocene catalysts display a quite rich insertion chemistry as well as C-H and C-C activation chemistry, flocks of organic and organometallic chemists have been lured into this field. Theoretical chemists also got involved and contributed much to shed light on the mechanistic behavior of these catalysts at the molecular level. In fact, the well-defined chemical structure of metallocenes offered the exceptional opportunity for the application of emerging computational methods in an effective manner. The elementary steps and the mechanism of stereocontrol of olefin polymerization by group 3 and 4 metallocenes probably represent the most thoroughly studied organometallic reactions. The outcome of almost 20 years of highly competitive and enthusiastic research has been multifold: the performance of metallocene and related catalysts has been enormously improved; an impressive number of new molecules and new reactions to make them have been produced; new polyolefin materials have been invented; and the understanding of the elementary steps involved in the reaction between olefins and transition-metal carbon bonds has been expanded considerably. The most successful and best studied metallocene catalysts are the chiral, C2-symmetric ansa-zirconocenes, for which a large number of insertion, isomerization, and chain release reactions have been documented in the polymerization of propene. Chiral, C2-symmetric ansa-zirconocenes are isospecific by virtue of their symmetry, producing isotactic polypropenes that, in comparison to Ti-based heterogeneous catalysts, have narrower molecular weight distributions, isotacticities spanning from almost atactic to perfectly isotactic, an often incomplete regioregularity (indicated by the detection of isolated secondary propene units), a random distribution of stereo- and regioerrors in the polymer chain, and lower molecular


weights, due to several facile β-hydride transfer reactions. Like any other active species involved in fast catalytic processes, metallocenes too obey Heisenberg’s uncertainty principle: we cannot watch them at work without modifying them, although remarkable progress has been made in determining what an active site actually looks like. In fact, a quite detailed and predictive understanding can be reached by peering at the active sites through the X-ray structure of the metallocene catalysts precursors, the molecular modeling of the hypothetical active species, and the microstructure and end group structure of the polymers made with them. Indeed, the polymer microstructure is largely the fingerprint of the catalyst that made it. The choice of metallocene-made polypropene as a subject worth a review stems from two considerations: the first is that, although quite a few reviews and monographies have appeared on the subject of metallocene-catalyzed polymerization of olefins,3,16,34-46 only two focused on polypropene;36,46 in addition, the catalytic polymerization of propene, although 45 years old, is still evolving at such a frantic pace that it needs to be reviewed quite often. Isotactic polypropene (i-PP) is produced in some 25 million tons per year with Ti/MgCl2-based heterogeneous catalysts and is one of the fastest growing polyolefins, both in terms of sheer production volume and in the number of applications.47-49 The fact that metallocene catalysts can give access to PP structures not previously attainable with conventional catalysis is giving even more impetus to this market. This review covers the homopolymerization of propene with group 4 metallocene catalysts, and special emphasis is dedicated to isotactic polypropene. Reference to other poly(1-olefins) and copolymerization between propene and minor amounts of ethene will be done only when relevant to the discussion. As sources of information we use here only the available scientific publications, and patent literature is cited only occasionally. This choice is due mainly to the fact that patent examples produce only very limited polymer characterization data. Views expressed on this review are those of the authors, which are not necessarily those of our employers.

II. Basic Concepts A. Structure of Group 4 Metallocenes As defined in the Introduction, group 4 metallocenes are d0, pseudotetrahedral organometallic compounds in which the transition metal atom bears two η5 cyclopentadienyl ligands and two σ-ligands (Chart 1). The two cyclopentadienyl ligands remain attached to the metal during polymerization (for this reason they are also referred to as “ancillary” or “spectator” ligands) and actually define the catalyst stereoselectivity and activity as we will describe in detail. One or both of the two σ-ligands are removed when the active catalyst is formed. Due to their aromaticity, cyclopentadienyl anions are six-electron donors and very robust ligands. The most commonly encountered cyclopentadienyl-type ligands are cyclo-

Resconi et al. Chart 1. General Structure of a Group 4 Bent Metallocene with the Most Relevant Anglesa

a Mt ) Ti, Zr, Hf; E ) R C, R Si, CH CH , etc.; X ) 2 e2 2 2 2 σ-ligand.

pentadienyl itself (C5H5-, or Cp), alkylated cyclopentadienyls such as pentamethylcyclopentadienyl (Me5Cp-, or Cp*), indenyl (C9H7-, or Ind), and fluorenyl (C13H9-, or Flu). Two good reference books on the synthesis, characterization, and reactivity of cyclopentadienyl ligands and group 4 metallocenes are available.25,27 The carbon atoms of the Cp ligands can bear hydrogen or other substituents such as alkyl, aryl, or silyl groups: up to 10 different substituents are possible on a metallocene, and this high structural diversity is the reason for the high steric and electronic versatility of the Cp ligands. Different substituents change not only the size and shape of the Cp ligands, but also the Cp-Mt-Cp distances and angles. This is shown in Table 1 for a series of different zirconocenes. All chemical transformations relevant to metal/ olefin reactions occur at the three orbitals in the plane between the two Cp rings (the “wedge” or belt). Although the electronic structure of bent bis(cyclopentadienyl) transition metal complexes has been investigated by several authors,52-55 the first detailed analysis of the electronic structure of group 4 metallocenes, and the implications on their chemistry, has been performed by Lauher and Hoffmann.56 Their analysis was based on the generic metallocene reported in Scheme 1, with eclipsed Cp rings and hence of C2v symmetry, and with the angle R equal to 136°. The metallocene equatorial belt is in the yz plane, and the C2 axis is along the z axis. As already pointed out by Brintzinger and Bartell,52 of the five frontier orbitals, the most important to the following discussion are the three low-lying 1a1, b2, and 2a1 orbitals reported in Figure 1. All three orbitals have significant extent in the yz plane, which corresponds to the plane defining the equatorial belt of the metallocene. The b2 orbital is chiefly dyz in character, while the two a1 orbitals in addition to contribution from the dx2-y2 and dz2 orbitals, contain s and pz contributions. The 1a1 orbital resembles a dy2 orbital and is directed along the y axis, while the 2a1 orbital is the highest in energy among the three orbitals and points along the z axis. Other authors previously reached similar conclusions.

B. Activation It is now well-established that the active polymerization species is a metallocene alkyl cation.57 By reaction of a metallocene dichloride or dialkyl (the



Table 1. Relevant Angles (deg) of Selected Cp′2ZrX2 Complexesa

a

See Chart 1 for definition of the listed angles.

Scheme 1a

a

Modified from ref 56.

stable, inactive precatalyst) and a suitable Lewis or Brønsted acid (whose conjugated base is a poorly coordinating anion), a very reactive, highly Lewis acidic cationic metal center is generated. Of the three metals, Zr is the most active, followed by Hf and Ti. The latter also suffers from deactivation at the higher temperatures, possibly because of reduction to TiIII. MAO is the most widely used cocatalyst, able to activate the largest number of metallocenes and other soluble complexes. It is obtained by the controlled hydrolysis of AlMe3, but its composition is far from

being known. Cryoscopic, GPC, and NMR studies have shown that MAO is a mixture of several different compounds, including residual (coordinated) AlMe3 and possibly AlO3 units, in dynamic equilibrium.58-62 The generally accepted mechanism of metallocene activation by MAO is shown schematically in Scheme 2. The true nature of the activating species in MAO has not been elucidated yet. An interesting and very detailed study carried out by Barron and co-workers63,64 on the hydrolysis products of Al(t-Bu)3 might give some important insight on the structure(s) of oligomeric methylalumoxane. In light of Lasserre’s and Barron’s results, dynamic cage structures are more likely than linear or cyclic structures. Recent DFT calculations of Zakharov and coworkers on models of MAO with structures (MeAlO)n, with n ) 4, 6, 8 and 12, also found that cage structures with n > 4 are more stable than cyclic structures.65 According to their calculations, the cage structures with n ) 6, 8, and 12 are more stable than


Resconi et al. Scheme 3

Scheme 4. A New Stereogenic Center Is Formed at Every Insertion of a Prochiral Olefin Such as Propene, into the Metal-Growing Chain Bond (Mt-P)a

a For example, a methine of S chirality is formed upon insertion of a re-coordinated propene.

Figure 1. Contour diagram in the yz plane of the three most important extended Hu¨ckel molecular orbitals of the generic bent metallocene Cp2Mt. Solid and dashed lines correspond to positive and negative contour of the wave function.56 Scheme 2

cyclic structures by roughly 15 kcal/mol of MeAlO unit. Main drawbacks of MAO as cocatalyst are its relatively high cost, due to the high cost of the AlMe3 parent compound; the large amount needed (typically Al/Zr ) 103-104 M are used, although in supported systems Al/Zr ratios as low as 100 M has proven sufficient); the high residual content of catalyst residues (alumina) in the final product, especially for systems of not very high activity, as it is often the case in propene polymerization; and the intrinsic danger connected to the use of extremely pyrophoric AlMe3. To solve the above problems, MAO surrogates have been investigated. Reports in the patent literature include the use of MAO/Al(i-Bu)3 mixtures66 or the hydrolysis products of Al(i-Bu)3 and other branched aluminum alkyls.67-69 A different strategy toward simpler and cheaper metallocene-based systems has been the use of boron compounds such as B(C6F5)3, NR3H+B(C6F5)4- and Ph3C+B(C6F5)4- in combination with metallocene dialkyls.46,70-76 As catalyst activation is discussed in detail in another review of this issue, we will not further discuss this point. We only add that, in most practical cases and

in order to have more reproducible results and save on the amount of cocatalyst, adding small amounts of AlR3 (such as Al(i-Bu)3 and AlEt3) to the reaction system is a common procedure to scavenge impurities and, in some cases, to alkylate the metallocene dichlorides.77,78

C. Elements of Chirality In this section we present the elements of chirality relevant to the stereospecific polymerization of propene with group 4 metallocenes. First of all, coordination of a prochiral olefin, such as propene, gives rise to nonsuperimposable coordinations.79 To distinguish between the two propene coordinations, we prefer the nomenclature re, sisdefined for specifying heterotopic half-spacess79 instead of the nomenclature R, Ssdefined for double or triple bonds π-bonded to a metal atoms80,81 in order to avoid confusion with the symbols R and S used for other chiralities at the same catalytic site, or the nomenclature Re, Sis defined for reflection-variant unitss82 and used by Pino and co-workers in refs 83-86. The use of the si, re nomenclature can be confusing when different monomers are considered, because the name of a fixed enantioface of an 1-olefin depends on the bulkiness of the substituent in position 1. However, since propene is the only monomer considered in this review, this problem does not exist here. We only remark that the re and si coordinations sketched in Scheme 3 correspond to the R and S coordinations, respectively. A second element of chirality is the configuration of the tertiary carbon atom of the growing polymer chain nearest to the metal atom. In fact, a new stereogenic center is formed in the growing chain at every propene insertion (Scheme 4). The standard Cahn-Ingold-Prelog R, S nomenclature81,82 can be used here. A third element of chirality is the chirality of the catalytic site, which, in particular, can be of two different kinds: (i) the chirality arising from coordinated ligands, other than the alkene monomer and the growing chain. For the case of metallocenes with


Chemical Reviews, 2000, Vol. 100, No. 4 1259 Scheme 5. Four Possible Insertion Modes of a Prochiral Olefin Such as Propene, into the Mt-Growing Chain (here simulated by a methyl group) Bonda

Figure 2. On the left, a model catalytic complex comprising a Me2C(1-Ind)2 ligand, a propene molecule re-coordinated and an isobutyl group (simulating a growing primary polypropene chain). The chirality of coordination of the bridged π-ligand is (R,R), labeled according to the absolute configurations of the bridgehead carbon atoms which are marked by arrows. On the right, a model catalytic complex comprising a Me2C(Cp)(9-Flu) ligand, a propene molecule re-coordinated and an isobutyl group. No chirality of coordination of the bridged π-ligand exists, while R is the chirality at the metal atom.

prochiral ligands, it is possible to use the notation (R) or (S), in parentheses, according to the CahnIngold-Prelog rules81,82 extended by Schlo¨gl.87 For instance, the (R,R) chirality of coordination of the Me2C(1-Ind)2 ligand, labeled according to the absolute configurations of the bridgehead carbon atoms (marked by arrows), is shown in Figure 2. (ii) An intrinsic chirality at the central metal atom, which for tetrahedral or assimilable to tetrahedral situations can be labeled with the notation R or S, by the extension of the Cahn-Ingold-Prelog rules, as proposed by Stanley and Baird.88 This nomenclature has been used to distinguish configurationally different olefin-bonded intermediates which may arise by exchanging the relative positions of the growing chain and of the incoming monomer.89-91 For instance, the model with intrinsic R chirality at the central metal atom is shown in Figure 2, for the case of a metallocene with a Me2C(Cp)(9-Flu) ligand. For the case of models with C1 symmetric metallocenes, we will mainly use the more mnemonic notation, according to which the relative disposition of the ligands that presents the coordinated monomer in the more (less) crowded region is referred to as “inward (outward) propene coordination”,92 although the extended nomenclature R, S could have been used as well. As it will be shown in detail in the following, one or both of these kinds of chirality at the catalytic site can be present in the models. For the case of model complexes in which two carbon polyhapto ligands are tightly connected through chemical bonds (the socalled bridge), which are then stereorigid (we shall call them ansa using the nomenclature introduced by Brintzinger), only the chirality of kind ii can change during the polymerization reaction. Since 1-olefins are prochiral, in principle they can coordinate and insert into a transition metal-carbon bond in four different ways (Scheme 5). Whether the olefin insertion is primary or secondary defines the regiochemistry of insertion (thus the catalyst regioselectivity and the regioregularity of the polymer),

a Primary propene insertion occurs when the CH group of the 2 olefin binds to the metal. Top views: the two coordination intermediates that will give rise to primary propene insertion. Secondary propene insertion occurs when the CH2 group of the olefin binds to the growing chain. Bottom views: the two coordination intermediates that will give rise to secondary propene insertion.

Scheme 6

while the choice of the olefin enantioface (or enantioface selectivity) defines the stereochemistry of each insertion (the catalyst stereoselectivity). The insertion of an 1-olefin into a metal-carbon bond is mostly primary (1,2), with a few exceptions that will be discussed in detail, for the case of metallocenes, in sections VII and IX. Since every propene insertion, whatever its orientation, creates a new stereogenic center, the catalyst stereoselectivity (and the stereoregularity or tacticity of the polymer) is determined by the stereochemical relationship(s) between the stereogenic carbon atoms in the polymer chain.

1260 Chemical Reviews, 2000, Vol. 100, No. 4 Scheme 7. Chain Segments Are Shown in Their trans-Planar and Modified Fisher Projections

Resconi et al.

Multiple insertions of the same enantioface produce a polymer chain with chiral centers of the same configuration, i.e., an isotactic polymer (A in Scheme 7). Multiple insertions of alternating enantiofaces produce a polymer chain with chiral centers of alternating configuration, i.e., a syndiotactic polymer (B in Scheme 7). Random enantioface insertions produce a polymer chain with no configurational regularity, i.e., an atactic polymer (C in Scheme 7). While both isotactic and syndiotactic polypropenes are partially crystalline materials with relatively high melting points (up to 160-170 °C for i-PP, and ∼150 °C for s-PP), atactic polypropene (a-PP) is a fully amorphous polymer, since it lacks long-range stereochemical regularity.

D. Polymerization Mechanism

In Ziegler-Natta catalysis, and in general in coordination polymerization, a polyolefin is produced by multiple insertion of olefins into a metal-carbon bond. Olefin insertion occurs by cis opening of the double bond (both new bonds are formed on the same side of the inserting olefin) and with chain migratory insertion (it is the alkyl group on the metalsthe growing chainsthat migrates to the olefin, with a net exchange of the two available coordination positions on the metal center). When insertion is primary, the 1-olefin enantioface which is inserted preferentially is the one which, in the transition state, places its substituent anti to the first C-C bond of the growing chain, since this arrangement minimizes nonbonded interactions (Scheme 6). Scheme 8a

a

Adapted from ref 102.

The key features of the insertion mechanism are that the active metal center bearing the growing alkyl chain must have an available coordination site for the incoming monomer, and that insertion occurs via chain migration to the closest carbon of the olefin double bond, which undergoes cis opening with formation of the new metal-carbon and carboncarbon bonds: the new C-C bond is then on the site previously occupied by the coordinated monomer molecule. So far, two main mechanistic schemes (1 and 2 in Scheme 8) have been proposed for olefin polymerization catalyzed by group 3 and 4 transition metals. The first of these mechanisms is named after Cossee7,93-97 and substantially occurs in two steps: (i) olefin coordination to a vacant site and (ii) alkyl migration of the σ-coordinated growing chain to the π-coordinated olefin. At the end of the reaction, a net migration of the Mt-chain σ-bond to the coordination position previously occupied by the olefin occurs. The second mechanism is due to Green and Rooney55,98


and involves an oxidative 1,2-hydrogen shift from the first C atom of the growing chain to the metal, giving rise to an alkylidene hydride species bonded to the metal. A four-center metallacycle is then generated by reaction of the alkylidene moiety with a coordinated monomer molecule. The final step is a reductive elimination reaction between the hydride species bonded to the metal and the metallacycle. The Green-Rooney mechanism is ruled out because 14e-, cationic d0 metallocenes are effective polymerization catalysts, and these complexes lack the required d electrons for formal oxidative addition. The last two mechanisms of Scheme 8 are improved versions of the Cossee mechanism. The third one (also referred to as “Modified Green-Rooney mechanism”) is due to Green, Rooney, and Brookhart99-101 and requires the presence of a stabilizing R-agostic interaction both in the ground-state olefin complex and in the four-center transition state. Finally, the fourth mechanism (very similar to the modified Cossee mechanism 3) requires the presence of an R-agostic interaction in the transition state only. Although differences do exist between the original and R-agostic assisted Cossee mechanisms, they all agree that monomer insertion is a two-step process, that is, coordination followed by insertion. Moreover, they concord that the active metal center bearing the growing alkyl chain must have an available coordination site for the incoming monomer and that the olefin insertion occurs by (i) cis opening of the double bond and (ii) with chain migratory insertion. These mechanisms also indicate that in order to undergo insertion, an olefin has to coordinate face-on to the metal, with its double bond parallel to the metalcarbon bond. Whether the metal-olefin complex is a real chemical species or the olefin undergoes direct insertion into the metal-carbon bond has been a matter of debate for many years. However, several experimental and theoretical studies of the last years have established that such species do exist,103-111 although as transient species, and are required to explain some kinetic evidence (see section VIII). A distinction must be made here between active center and active site: a metallocene-type active center (or active species) has a minimum of two sites (the two tetrahedral positions previously occupied by the two σ-ligands of the metallocene precatalyst) on which chain growth can take place. The nature of the active site is determined by the metal, the Cp ligands geometry, and the structure of the metal-bonded chain end. Thus, the different types of last inserted monomer unit (primary or secondary, re or si face) increase the number of possible active sites. The different sites can be different in reactivity, regioselectivity, and enantioface selectivity, and as a result, the active center itself changes during a single chain growth but statistically behaves in the same way from one polymer chain to the next. Therefore, such a species is a single-center catalyst. If only one site can coordinate the olefin, then there is a limited number of polymer microstructures that can be obtained (Scheme 9). Metallocene catalysts allow formation of virtually any polyolefin structure because of the two-site, chain

Chemical Reviews, 2000, Vol. 100, No. 4 1261 Scheme 9. The Key-in-the-Lock Model: One Lock, One Keya

a If the site is nonselective at all (top), propene can insert in four different ways, giving rise to a regio- and stereoirregular polypropene. If the site is regioselective but not stereoselective (middle), propene can insert in two different ways only, corresponding to nonselective primary insertion, giving rise to a regioregular but atactic polypropene. If the site is both regioselective and stereoselective (bottom), propene can insert one way only, giving rise to a regioregular and isotactic polypropene.

migratory insertion with site-switching mechanism, shown in Scheme 10. The relationship between metallocene site symmetry and polymer stereochemistry has been fully understood. We can visualize the general mechanism for enantioface selectivity, in the chain migratory insertion with site switching operating with metallocene catalysts (enantiomorphic site control), using the key-in-the-lock formalism with two locks. Every active metal atom has two available coordination sites (the two locks) that can both insert the olefin and that can be different in either shape or chirality. Because of site switching, the monomer has to be inserted alternately on each site. For metallocenes, pathway A is the rule, while pathway B is an occasionally skipped insertion, or it can be a competing pathway only for some highly asymmetric ligands such as, for example, in the aspecific meso-C2H4(1-Ind)2ZrCl2.92 It is important to note that there are some differences between metallocenes and the available models for heterogeneous, Ti-based, catalysts: according to Arlman and Cossee,93-96 an octahedric, stereoselective Ti on the surface of a crystalline lattice has only


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Scheme 10. The Key-in-the-Lock Model: Two Locks, One Keya

a Every active metal atom has two available coordination sites (the two locks), which can both insert the olefin, and that can be different in either shape or chirality. In the framework of the chain migratory insertion mechanism, the monomer has to be inserted alternately on each site, and the structure of the resulting polypropene depends on the combination of the regio- and enantioselectivity of the two active sites.

one site able to coordinate and insert the olefin. The growing chain, which has to migrate to the site previously occupied by the olefin in order to allow for insertion, then goes back to its former position, that is, strictly following pathway B (chain back-skip at every insertion). As a result, and because the ligands around the Ti atom cannot be modified at will, these catalysts are, in practice, able to produce either isotactic or atactic polypropene only (Scheme 9). On the other hand, the presence of two active polymerization sites on the same metal center of metallocene catalysts, sites that can be different in shape or symmetry, allow for a much larger set of possible polymer microstructures than with any other catalyst. In addition, as metallocenes are discrete molecules whose molecular structures can be studied in detail, the shape of the active sites can be tuned in order to obtain the desired type and degree of stereoregularity. Finally, this fine-tuning can also be done on the chain release reactions (see section III.F), thus allowing a high degree of molecular weight control. In summary, the most important differences between metallocene and heterogeneous, Ti-based polymerization catalysts are (i) metallocenes are soluble, well-characterized, and homogeneous (in the sense of chemical composition), while heterogeneous Ti catalysts contain a wide variety of active centers. (ii) Because of the above chemical homogeneity, a large fraction of the metal atoms is active in metallocene-based systems, compared to only a small fraction (usually less than 1%) of surface Ti atoms in TiCl3 or supported TiCl4 catalysts. (iii) Metallocene cations are pseudotetrahedric, while surface Ti is octahedric (Scheme 11). The mechanism of chain growth is different for the two classes of catalysts, with two sites available for coordination-insertion on metallocene centers, and only one on surface Ti

Scheme 11

atoms. (iv) According to the most recent models, stereoselective surface Ti atoms have local C297 or isospecific C1 symmetry (hence can produce isotactic polymers only), while metallocenes can be of C1, C2, and Cs symmetry; in addition, the ligand environment (in terms of both sterics and electronics) in metallocenes can be changed to a very large extent.

E. Mechanisms of Stereocontrol in Primary Insertion (Site vs Chain-End Control) There are two possible sources of enantioface selectivity in olefin insertion. The most effective one is the stereogenicity of the metal active site; in this case, the mechanism of stereoselection is referred to

Propene Polymerization with Metallocene Scheme 12

as enantiomorphic site control, that is, the chiral induction comes from the asymmetry of the reaction site. It is the chirality relationship of the two coordination sites of the catalytic complex that determines the stereochemistry of the polymer. We have also seen that every monomer insertion generates a new stereogenic center. As a consequence, chiral induction (that is, enantioface preference) can come from the last unit, and this mechanism is referred to as chain-end control (Scheme 12). Hence, there can be four stereospecific polymerization mechanisms in primary polyinsertion, all of which have been documented with metallocene catalysts (Scheme 13): the two originated by the chiralities of the catalyst active sites, referred to as enantiomorphic site control (isospecific22 and syndiospecific112,113 site control), can be relatively strong, with differences in activation energy (∆∆E‡) for the insertion of the two enantiofaces up to 5 kcal/mol. A value of 4.8 kcal/mol has been found by Zambelli and Bovey for a Ti-based heterogeneous catalyst.114 Because of the mechanism of enantioface selectivity and the two-site, chain migratory insertion mechanism, the microstructure of a poly(1-olefin) made with a given metallocene is, to a large extent, predictable. In a series of landmark papers, Ewen and coworkers22,46,112,113,116-118 and Kaminsky and co-workers23 described a series of stereoselective metallocene catalysts which define what are now referred to as “Ewen’s symmetry rules”. These are summarized in Chart 2. When the metallocene molecule is C2v, meso Cs-symmetric, or highly fluxional, an aspecific polymerization has to be expected.

Chemical Reviews, 2000, Vol. 100, No. 4 1263 Chart 2. Steric Control as a Function of Metallocene Symmetry (Ewen’s Symmetry Rules)a

a

E ) enantioselective site; A ) nonselective site.

The most important cases of symmetry-related polymerizations (that is based on the mechanism of site control) of propene are discussed in sections III and IV. When the two sources of chiral induction are either absent or too weak to be effective, atactic or nearly atactic polyolefins are produced. Truly atactic polypropene is produced by two types of metallocenes: Achiral, C2v-symmetric unbridged (e.g. Cp2ZrCl2) and by extension any alkyl-substituted metallocene lacking stereorigidity (e.g. (MeCp)2ZrCl2,

Scheme 13. Mechanisms of Stereocontrol in Primary 1-Olefin Polyinsertiona

a If the enantiomorphic site control is operative (top-half view), stereoerrors do not propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of rr and mm triads, respectively. If chain-end control is operative (bottom-half view), stereoerrors propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of isolated r and m diads, respectively. Reprinted from ref 115. Copyright 1992 American Chemical Society.

1264 Chemical Reviews, 2000, Vol. 100, No. 4 Scheme 14

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type and amounts of regioerrors on the ligand structure and the polymerization conditions. The available data are discussed in sections VII.C-E. The lower reactivity of a secondary growing chain with respect to a primary growing chain has been confirmed in three ways: by studying the activating effect of hydrogen (see section IX), by copolymerization with ethene,126-129 and by end group analysis.130,131 The latter two aspects are discussed in section VII.B. In section VII.F we discuss the proposed mechanisms of isomerization of a secondary growing chain into a 3,1 unit.

G. 13C NMR Analysis of Polypropenes Ind2ZrCl2) as well as bridged, stereorigid, C2v-symmetric metallocenes (e.g. Me2Si(Cp)2ZrCl2 or Me2Si(Me4Cp)2ZrCl2)115,119 and the achiral, meso isomers of ansa-metallocenes, such as meso-C2H4(1-Ind)2ZrCl2, and meso-C2H4(4,5,6,7-H4-1-Ind)2ZrCl2.120,121 High molecular weight atactic polypropene has been produced with C2H4(9-Flu)2ZrCl2122 and Me2Si(9-Flu)2ZrCl2.123,124 Since the synthesis of atactic polypropene with metallocene catalysts has been reviewed elsewhere,125 it will not be further discussed here.

F. Regiochemistry of Propene Insertion Olefin insertion into metallocene Mt-C bonds is largely predominantly primary. However, one of the features of most isospecific metallocene catalysts is their generally lower regioselectivity compared to heterogeneous Ziegler-Natta catalysts: indeed, despite the fact that primary propene insertion is clearly favored by electronic factors (see section III.E), isolated secondary propene units are often detectable in i-PP samples and their presence is the signature of a metallocene catalyst. Tail-to-head propene insertions, currently referred to as secondary or 2,1 insertions, occur in i-PP from isospecific metallocene catalysts with high but opposite (with respect to primary insertions) enantioface selectivity (Scheme 14). These regiodefects have a strong effect in lowering crystallinity and melting point of i-PP. At the same time, there is also a close correlation between catalyst regioselectivity on one side and catalyst activity and polymer molecular weight on the other, due to the lower monomer insertion rate at a secondary growing chain end, and the competing β-H transfer to the monomer after a secondary insertion. Because of these two aspects, understanding the factors controlling the regioselectivity of a metallocene catalyst is important for catalyst design. The characterization of the different regioerrors by 13C NMR and the molecular modeling studies performed on the subject are discussed in section VII.A. The relative amounts of the different regiodefects are highly dependent on the metallocene ligand structure and the polymerization conditions employed (polymerization temperature and monomer concentration). Unfortunately, possibly due to the often low concentration of regioerrors and the requirement of a high-field NMR instrument and long acquisition times, few detailed studies have been carried out on the regioselectivity of metallocene catalysts and the dependence of the

The most powerful (if not the only) tool for the determination of the microstructure of polyolefins and the polymerization mechanism is solution 13C NMR.132-135 In the case of polypropene, the chemical shift of the methyl groups is highly sensitive to the relative stereochemistry of neighboring monomer units, that is, each methyl C has a different chemical shift depending on the configuration of the adjacent methynes, up to five on each side (a sequence length of 11 consecutive monomer units). Usually, statistical analysis is done at the pentad (sequence length of five consecutive monomer units) level. The degree of isotacticity can be given as the pentad, triad, or diad content (% mmmm, % mm, % m, respectively, Chart 3). For polyolefins of low stereoregularity, the degree of iso (syndio) tacticity is better given as the diad excess, % m - r (% r - m).115 Isolated insertion errors (as both secondary units or opposite enantiofaces) are easily and quantitatively detected by 13C NMR analysis, and triad/pentad analysis gives unambiguous identification of the polymerization mechanism. Some useful relationships for the four stereospecific polymerization mechanisms discussed above (see Scheme 13) are (a) isospecific site control: main peak, mmmm; misinsertions, [mr] ) 2[rr]; [mmmr] ) [mmrr] ) 2[mrrm]; (b) syndiospecific site control: main peak, rrrr; misinsertions, [mr] ) 2[mm]; [rrrm] ) [mmrr] ) 2[rmmr]; (c) isospecific chain-end control: main peak, mmmm; misinsertions, mr only; [mmmr] ) [mmrm]; (d) syndiospecific chain-end control: main peak, rrrr; misinsertions, mr only; [rrrm] ) [rrmr]. Site control is identified by the relationships 2[rr]/[mr] ) 1 (isospecific) and 2[mm]/ [mr] ) 1 (syndiospecific). Chain-end control is identified by the relationship 4[mm][rr]/[mr]2 ) 1. The average block length is obtained as 2[m]/[r] + 1 (isospecific triad test) and 2[r]/[m] + 1 (syndiospecific triad test). The proton spectra (400 MHz, C2D2Cl4, 120 °C) of i-PP and s-PP are compared in Figure 3. The methyl pentad region of the 100 MHz 13C NMR spectra of atactic, isotactic, and syndiotactic polypropene is shown in Figure 4a. By comparison with the methine (Figure 4b) and methylene (Figure 4c) regions of the 13C NMR spectra, it is evident how much more stereochemical information can be extracted from the methyl resonances.

III. Elementary Steps Many quantum mechanics studies have been devoted to clarifying the elementary steps of olefin



Chart 3. Nomenclature and Symmetry of Stereosequences in Polypropenea

a

m ) “meso” diad, r ) “racemic” diad, i ) isotactic triad mm, h ) heterotactic triad mr, s ) syndiotactic triad rr.

coordination and insertion in systems based on the simplest Cp2Mt metallocenes, bridged or not. Since propene increases considerably the number of situations to be studied (propene can coordinate in four different ways, while ethene just in one), and the insights gained into the elementary coordination and insertion steps would not be much deeper, the largest amount of these investigations used ethene as monomer, and only a few of them considered propene. For this reason, the following sections regarding the coordination and insertion steps will mainly focus on ethene. As the topic becomes the mechanism of stereocontrol, obviously propene and chiral ligands (hence more complex than Cp2) have to be considered. So far, all studies regarding the enantioselectivity in primary or secondary insertion, as well as the effect of ligand substitution on both enantioselectivity and regioselectivity, have been accomplished by using the molecular mechanics approach. However, the recent development of combined quantum mechanics/molecular mechanics, QM/MM, techniques certainly represents one more weapon in the armory of computational chemists.137-145 This technique merges the accuracy of the QM methods in describing the reactive part of the systemsbreaking/forming of bondss with the computational advantages of the MM meth-

ods in describing the steric effect due to the catalyst ligands.

A. Alkene-Free Species The position of a single σ-bonding ligand is extremely relevant to homogeneous polymerizations. In fact, the destiny of the growing chain at the end of each insertion stepsi.e., whether it remains in the position previously occupied by the monomer, or it is free to switch between the two coordination positions, or it is preferentially oriented along the local symmetry axis relating the two Cp ringssis fundamental in determining the stereoselective behavior of C1 and Cs-symmetric catalysts. This will be discussed in more detail in section IV. To understand the geometries assumed by the growing chain in the absence of a further ligand (e.g. counterion, solvent, monomer) we have to understand the interactions between a simple σ-bonding ligand with the bare Cp2Mt skeleton. With the usual extended Hu¨ckel molecular orbitals analysis, Hoffmann elegantly showed that a simple σ-bonding ligand, as H-, which approaches the d0 metallocene skeleton along the z axis (corresponding to the local symmetry axis in Scheme 15) will interact very well with the high-energy 2a1 orbital (see Figures 5 and 1), some-


Figure 3. 1H NMR (400 MHz, C2D2Cl4, 120 °C) of i-PP (top) and s-PP (bottom). Assignments of the diastereotopic methylene protons in i-PP are according to ref 136.

what with the 1a1 orbital, and not at all with the b2 orbital.56 If a different geometry is adopted, one with the H- ligand forming an angle R * 0° with the z axis, stabilizing interactions of the σ-orbital of the H- ligand with the low-lying 1a1 and b2 orbitals of the metallocene can be obtained. For the hypothetical Cp2TiH+ system, the energy minimum is calculated to come at about R ) 65°. Other authors subsequently revisited these conclusions at higher levels of theory, which should provide more reliable energetics. However, the results are not clear-cut. As for the simple model systems of the type Cl2TiCH3+ and H2TiCH3+, calculations based on classical ab initio,146-150 GVB,151 DFT,148,150 and toplevel CCSD(T) methods150 are in agreement with the conclusions of Hoffmann. As the models include the more representative Cp rings, the results obtained with different methods are contradictory. If the σ-ligand is H-, all the reported calculations are in agreement with results of Hoffmann.151-154 On the contrary, when the σ-ligand is CH3-, the HF and MP2 calculations of Morokuma on H2Si(Cp)2Mt(CH3)+ (Mt ) Ti, Zr, Hf),153,155 the Car-Parrinello molecular dynamics symulations of Meier on the same Ti system,156 and the HF calculations of Ahlrichs on the Cp2Ti(CH3)+ system149 suggested that the CH3 group is oriented along the symmetry axis, although in the crystalline structure of [1,2-(CH3)2C5H3)]2Zr(CH3)+‚ CH3B(C6F5)3- the methyl group is clearly off-axis.75 Morokuma and co-workers suggested that the off-axis orientation of the methyl group (see Scheme 15) in the crystalline structure could be due to the presence of the negative counterion. With the methyl group off-axis, a better electrostatic interaction between the two charged ions could be obtained. On the contrary, the MP2 and DFT calculations of Ahlrichs on the Cp2-

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Ti(CH3)+ system149 and the DFT calculations of Ziegler on the Cp2Ti(CH3)+ and H2Si(Cp)2Zr(CH3)+ systems152 suggested that the CH3 group is off-axis oriented. The GVB calculations of Goddard and co-workers, moreover, showed that the value of the Cp-Mt-Cp bending angle R influences the relative stability of the on- and off-axis geometries.151 The on-axis geometry is favored by larger R values, due to an increased steric pressure of the Cp rings on the R group, which clearly favors the on-axis geometry. A systematic study by Ziegler and co-workers on various d0 systems of the type L2MtCH3n+ (n ) 0, 1), where Mt is a group 3 or 4 metal atom and L is CH3, NH2, or OH,157 suggested an increased preference for the off-axis conformation as one moves down within a triad. This result was explained by a reduced steric pressure of the L ligands (which favors the on-axis geometry) bonded to a big metal at the bottom of the triad. Moreover, the energy of the 2a1 orbital, which is responsible for on-axis bonding, increases along the triad and therefore the preference for the off-axis geometry is enhanced.157 As for d0 group 3 metallocenes, Goddard151 and Ziegler157,158 and their co-workers found on-axis geometries for the Cp2ScH,151 Cp2ScCH3,158 and L2MtCH3157 (L ) CH3, NH2, OH) species. The preferential on-axis geometry for all the neutral Sc species was ascribed to the higher s orbital contribution to bonding for group 3 metals with respect to group 4 metals.151,157 Finally, it is worth noting that all the above studies have shown that the potential energy surface corresponding to the swing motion of the σ bonding ligand in the equatorial belt of the metallocene is very shallow. In all cases, the favored geometry, either onor off-axis, is favored by no more than 5 kcal/mol when Cp rings are present. As an alkyl group longer than a simple methyl group is σ-bonded to the metal atom, the situation is different, due to the possible formation of β- and γ-agostic bonds. With group 4 metallocenes, all authors substantially found an off-axis geometry when a β- or γ-agostic bond is present. However, the systematic study of systems of the type L2MtC2H5n+ (n ) 0, 1), where Mt is a group 3 or 4 metal atom and L is CH3, NH2, or OH, performed by Ziegler and co-workers, showed that the β-agostic bond only weakly perturbs the potential energy surface, which substantially remains similar to those present in the systems L2MtCH3n+.157 Finally, it is clear that the presence of a γ-agostic bond favors off-axis geometries, since the on-axis geometry would push the C atom which participates in the γ-agostic interaction toward the Cp rings.

B. Agostic Interactions As we will see, agostic interactions102,159-161 are almost ubiquitous in Ziegler-Natta catalysis. The X-ray structures of (MeCp)2Zr[(Z)-C(Me)dC(Me)(nPr)](THF)+ and of (MeCp)2Zr(C2H5)(PMe3)+, obtained by Jordan and co-workers,162,163 represent typical examples of such interactions. In both cases, see Figures 6 and 7 short Zr-Cβ and Zr-Hβ distances



Figure 4. The 100 MHz 13C NMR spectra of isotactic (top), atactic (middle), and syndiotactic (bottom) polypropenes: (a) methyl pentad region, (b) methine pentad region, and (c) methylene pentad region.


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Figure 5. Interaction diagram for the generic d0 bent metallocene Cp2Mt, on the left, and H-, on the right. The orbitals are sketched in the yz plane.56 Scheme 15

Figure 6. X-ray structure of (MeCp)2Zr[(Z)-C(Me)dC(Me)(n-Pr)](THF)+. Reprinted from ref 162. Copyright 1989 American Chemical Society.

(about 2.7 and 2.2 Å, respectively) are observed, together with a small Zr-CR-Cβ angle (about 90°). Similar agostic interactions were also reported by other authors.164-166 The nature of the agostic interaction was studied by Morokuma, with ab initio calculations on the Ti(C2H5)(PH3)2Cl2H complex.167,168 An agostic interaction mainly consists of a delocalization of electron density from the C-H σ-bonding orbital to empty d orbitals of the metal. The agostic interactions most important to Ziegler-Natta polymerization by group 4 metals are the R-, β-, and γ-interactions from the corresponding CR-H, Cβ-H, and Cγ-H σ-bonding orbitals. The geometry of n-butyl groups showing R-, β-, and γ-agostic interactions are shown in Figure 8. Usually, the most stable geometry corresponds to the β-agostic, followed by the γ and then the R ones. As examples, for the H2Si(Cp)2Zr(n-propyl)+ system Morokuma and co-workers calculated the γ-agostic geometry to lay 2.0 kcal/mol above the β-agostic one,153 while for the Cp2Zr(n-propyl)+ system, Ziegler and co-workers calculated that the γ- and the R-agostic geometries lay 6.4 and 11.2 kcal/mol above the β-agostic one.169 Although the agostic interactions are mainly due to donative interaction from the C-H bond, Morokuma146 and Ziegler169 noticed that some donation can occur from the C-C σ-bonding orbitals as well. The two molecular orbitals sketched in Figure 9 indicate such donation from both the CR-Cβ and CβCγ bonds.146

Figure 7. X-ray structure of (MeCp)2Zr(C2H5)(PMe3)+. Reprinted from ref 163. Copyright 1990 American Chemical Society.

C. Olefin Coordination The electronics behind olefin coordination to group 4 cationic L2MtR+ species has been studied in details by Marynick, Morokuma, and co-workers.146,170 Their analysis indicates that while the Mt-R bond in the Cs-symmetric Cl2TiCH3+ species chiefly involves a metal orbital which corresponds to the 1a1 orbital of Figure 1, the olefin coordination is due to in-phase interactions between the olefin π-orbital with metal orbitals corresponding to the 2a1, mainly, and to one lobe of the 1b2 orbitals of Figure 1. A good overlap between the olefin π-orbital and these metal orbitals



Figure 8. Orientations of the n-butyl alkyl group in Cp2Zr(n-butyl)+. Relative energies in kJ/mol, distances in pm, angles in deg.169

Figure 10. Orbital interaction diagram for coordination of the ethene fragment (on the right) to the Cl2TiCH3+ fragment (on the left). The MOs of the full TiCH3(ethene)+ complex are depicted in the middle. Only the most important MOs are included.146

Figure 9. Contour maps of occupied molecular orbitals (MO) showing donative interaction from the Cβ-Cγ and Cγ-H bonds (MO 31) and from the CR-Cβ and Cβ-Cγ bonds (MO 32).146

is obtained also when the olefin is rotated by 90°, to assume a geometry in which the C-C double bond is perpendicular to the equatorial belt of the metallocene. This implies a small electronic barrier to olefin rotation. Finally, since group 4 cations contain d0 metals, no back-bonding from the metal to the olefin π*-orbital is present. The orbital interaction diagram depicted in Figure 10 shows the most important orbitals involved in the coordination of ethene to the Cl2TiCH3+ system. The main interaction occurs between the lowest vacant orbitals of the TiCH3+ fragment, MOs 21a′ and 22a′, which resemble the MOs 2a1 and b2 of Figure 1, with the doubly occupied π-orbital of the ethene fragment, 6a′. The olefin uptake energy to alkene-free group 4 metallocenes of the type Cp2Mt(alkyl)+ has been calculated by several authors. When the alkyl group is the simple methyl group, olefin coordination usually occurs in a barrierless fashion, and uptake energies in the range 15-30 kcal/mol (depending on the particular computational approach/metallocene considered) have been calculated.149,152,153,171 There are a few exceptions to this general behavior. For example, in the coordination of ethene to the Cp2-

TiCH3+ system, computed at correlated MP2 level by Ahlrichs and co-workers, the olefin complex is not a stable species and directly inserts into the Ti-methyl σ-bond.149 Also, the coordination of ethene to systems of the type Cp2Mt(alkyl)+(CH3ClAl[O(Al(CH3)3AlHCH3]2)-sthe last fragment simulating MAOshas been calculated by Fusco and co-workers: according to their analysis, the formation of an ethene complex with the olefin sandwiched between the Cp2Mt(alkyl)+ and (CH3ClAl[O(Al(CH3)3AlHCH3]2)- fragments is unfavored by roughly 5-10 kcal/mol.172,173 Finally, according to Rytter, Ystenes, and co-workers,171 ethene coordination to the bulky (Me5Cp)2ZrCH3+ system requires the overcoming of a small energy barrier, essentially due to repulsive interactions between the olefin and methyl groups of the Me5Cp ligands, and the olefin uptake energy is only 2-3 kcal/mol. For neutral d0 scandocenes, the interaction between the olefin and the metallocene is reduced due to the absence of the favorable electrostatic cation-olefin interaction.152,174 As a consequence, ethene uptake energies have been calculated to be roughly 20 kcal/mol lower than the corresponding uptake energies for the analogous cationic group 4 metallocene. For group 4 metallocenes, ethene uptake energies in the range 5-10 kcal/mol have been calculated when alkyl groups longer than methyl are bonded to the metal atom and β- or γ-agostic interactions are present.152,169,171 The olefin uptake still is a barrierless process, unless bulky ligands as Me5Cp rings are considered.171 The substantially lower uptake energy


values calculated in the presence of alkyl groups longer than methyl are ascribed to the presence of a β- or γ-agostic interaction that stabilizes the alkene free metallocene. Propene has been found to interact better than ethene with the metallocene, since slightly higher uptake energies, roughly 2-3 kcal/mol, have been calculated for coordination of a propene molecule.175 However, steric hindrance can be particularly relevant. As an example, the propene uptake energy to the unencumbered H2C(1-Ind)2Zr(isobutyl)+ β-agostic system amounts to 12.7 kcal/mol, whereas the presence of the bulky tert-butyl group in the H2C(3-t-Bu-1-Ind)2Zr(isobutyl)+ β-agostic system reduces the propene uptake energy to 6.6 kcal/mol only.175 Although olefin uptake energies close to 10 kcal/ mol were calculated, the coordinated olefin has a quite high mobility. As previously discussed in terms of molecular orbitals, rotation of the coordinated olefin around the axis connecting the metal atom to the center of the C-C double bond is easy. This is suggested also by the static ab initio calculations of Morokuma and co-workers, which calculated the barrier for olefin rotation in the system Cl2TiCH3(ethene)+ to be lower than 1 kcal/mol,146 and by the first principles molecular dynamics simulation of the Cp2ZrC2H5(ethene)+ system by Ziegler and co-workers.176 The latter simulation also indicated that the olefin is quite capable of dissociating from the metal atom at room temperature. Before concluding this section, it has to be remembered that all the above calculations have been obtained by neglecting solvent effects. For cationic group 4 metallocenes, the solvent/metallocene interaction is mainly electrostatic (as the olefin/metallocene interaction). Therefore, it is reasonable to expect similar solvent/metallocene and olefin/metallocene coordination energies and a small barrier due to solvent displacement.177 Moreover, the uptake energy values only represent a contribution to the total free energy of coordination. In fact, an always unfavorable uptake entropy has to be accounted for. Although few experimental data are available, it is reasonable to assume that the -T∆S contribution to the free energy of olefin coordination to group 4 metallocenes at room temperature is close to the 10 kcal/mol value observed at 300 K for Ni and Pd compounds.178 The few computational data also suggest a -T∆S contribution close to 10 kcal/mol.157,179 As a consequence, olefin uptake energies higher than 10 kcal/mol are required to form stable olefin complexes in the gas phase. Again, the picture is quite different in solution, since olefin coordination probably requires the displacement of a coordinated solvent molecule. The entropy loss due to the olefin coordination could be counterbalanced by the entropy gain due to the dissociation of a coordinated solvent molecule. In conclusion, it is reasonable to expect that coordination/dissociation of the olefin from the metallocene is a process with a low energy barrier and with low energy gain/loss. Experimentally, examples of olefin adducts of d0 metallocenes are scarce. Moderately stable olefin adducts have been obtained when the olefin is

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tethered to the metal105-108,180 or to the Cp ligands.111 The experimental ∆G‡ values for metal olefin dissociation are close to 10 kcal/mol.105,107,111 Probably, the presence of the tether reduces strongly the entropy gain that favors the olefin dissociation, inducing the so-called “chelation effect”. Upper bounds to the olefin uptake energy can be obtained by measurements of the π-σ-π processes in fluxional allyl derivatives of group 3181 and group 4182 metallocenes. Again, ∆G‡ values close to 10 kcal/mol were observed. Systematic studies on olefin coordination to transition metals (not being part of a metallocene, though) have been reported by Siegbahn,183 Bauschlicher,184 and Ziegler157 and their co-workers.

D. Insertion The insertion reaction of a coordinated olefin into the Mt-C σ-bond, where Mt is a group 4 metallocene, or a model of it, has been the subject of several theoretical studies.56,146-150,152,153,156,169-171,174,176,177,185-190 All authors agree that the insertion reaction occurs through a slipping of the olefin toward the first C atom of the growing chain and that the four-center transition state assumes an almost planar geometry. In Figures 11 and 12, the geometry of the transition states for ethene insertion into the Zr-CH3 bond of the H2SiCp2ZrCH3+ and Cp2ZrCH3+ systems, calculated by Morokuma153 and Ziegler,152 respectively, are presented. Although obtained with substantially different computational approachessHartree-Fock for H2SiCp2ZrCH3+ and DFT for Cp2ZrCH3+sthe similarities between the two structures are quite strong. The main features of the geometries reported in Figures

Figure 11. Hartree-Fock optimized structure of the transition state for insertion of ethene into the Zr-CH3 bond of the system H2Si(Cp)2ZrCH3+.153

Figure 12. DFT optimized structure of the transition state for insertion of ethene into the Zr-CH3 bond of the system Cp2ZrCH3+.152


Figure 13. Molecular orbital diagram of the mixing process involved in the insertion of olefin into a metalcarbon bond. Orbital occupations are shown for the formal d0 configuration on the metal.191

11 and 12 indicate a very asynchronous transition state. In fact, in both structures the Zr-C bond that is going to be formed is only 5-10% longer than in the products, while the other bond being formed, the new C-C bond, is roughly 40% longer than in the products. Finally, the Zr-C bond that is going to be broken is only 5% longer than in the reactants, while the ethene C-C bond distance is quite closer to the value it has in the reactants than in the products. All these observations indicate an early transition state of very tight geometry. For a model based on the analogous group 3 Cp2ScCH3 system, the transition state is only slightly more advanced relative to the one for the cationic zirconocene.152,174 The electronics behind the insertion reaction is generally explained in terms of a simple three-orbital four-electron scheme. Hoffmann and Lauher early recognized that this reaction is an easy reaction, indeed, for d0 complexes, and they also recognized the relevant role played by the olefin π*-orbital in determining the insertion barrier.56 According to them, the empty π*-orbital of the olefin can stabilize high-energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the back-donation to the olefin π*-orbital.56 Since for d0 systems this back-donation does not occur, d0 systems were predicted to be barrierless, whereas a substantial barrier was predicted for d2 systems.56 A similar picture has been suggested by the DFT calculations of Ziegler and co-workers through a systematic study of the chain propagation reaction by complexes with d0 and d0fn transition metals.191 Their discussion is based on the MO diagram shown in Figure 13. In agreement with Hoffmann and Lauher, for d0 systems the lowest unoccupied molecular orbital (LUMO) of the olefin complex chiefly corresponds to a bonding olefin π*-d metal interaction. In the transition state, the occupied sp3 orbital of the first C atom of the growing chain (the one bonded to the metal) and the occupied π-orbital of the olefin form an energetically unfavorable bonding/ antibonding combination. Now, if the empty π*-


Figure 14. Dominant pairs of interacting orbitals in the transition state for ethene insertion into the Ti-methyl bond of the Cl2TiCH3+ system. Adapted from ref 185.

orbital of the olefin mixes in, the antibonding character of the highest occupied molecular orbital (HOMO) is transformed in a substantially more stable, nonbonding HOMO, while the energy of the LUMO rises due to the π-π* mixing. Again, for d0 systems, the insertion reaction is substantially barrierless, since the LUMO energy does not contribute to the total energy, whereas for d2 systems it is not, since for these systems the HOMO corresponds to the high-energy LUMO of d0 systems. To quantify this point, Ziegler and co-workers compared the insertion barrier for ethene insertion into the cationic d0 Cp2TiC2H5+ and neutral d1 Cp2TiC2H5 systems. The insertion barrier for the neutral d1 system is roughly 20 times higher than the insertion barrier for the cationic d0 system.191 Finally, it is worth noting that Hoffmann and Ziegler predicted that d1 and d2 complexes can be suitable polymerization catalysts if other ligands can accept the d electrons in orbitals orthogonal to the π* olefin orbital, which corresponds to a reduction of the relevance of the d-π* interaction,56,191 while Ziegler also noted that if the occupied metal d orbitals are lower in energy, e.g. for late transition metals, the destabilization due to the disruption of the metal to olefin π* orbital backdonation is smaller, and hence low insertion barriers are again possible.191 Fujimoto and co-workers analyzed the main orbital interaction at the transition state for ethene insertion into the Ti-C bond of the complex Cl2TiCH3+,185 with the paired interacting orbital method.192 The most important fragments are reported in Figure 14. The φ1′, Ψ1′ and φ2′, Ψ2′ pairs are responsible for the formation of the new Ti-C and C-C bonds, respectively. The contribution of the olefin π*-orbital to the Ψ1′ and Ψ2′ fragment orbitals is striking. A similar analysis was performed by Shiga and co-workers,186 which in agreement with the analysis of Hoffmann56 also predicted that ethene insertion is facile for d0 complexes whereas it is not with d2 complexes, and by Morokuma and co-workers.146 Similar mixing of the olefin π* was also observed by Jolly and Marynick.170 The presence of a favorable R-agostic interaction which stabilizes the transition state is another point of convergence between various authors.146,148-150,152,153,156,169,171,174,176,177,187-190 Before continuing, it is worth noting that a short Zr-H(R) distance (indicative of an R-agostic interaction) is


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Figure 15. A comparison between hyperconjugation and R-agostic interactions.102

Figure 16. Relative energies of the transition state (continuous line) and of the olefin complex (dashed line) for propene insertion into the Zr-methyl bond of the H2Si(Cp)2ZrCH3+ system, as a function of the Zr-H(R) distance.175

almost inevitable as the sp3 orbital of the chain C atom bonded to Zr tilts away from the Zr-C axis to be oriented toward the closest C atom of the olefin, giving rise to the bonding interactions with the olefin itself. The energy changes along the reaction path for insertion of ethene into the Zr-methyl bond of the Cp2ZrCH3+ system, with normal and deleted Zr-H(R) overlap integralssthat is, considering or not the agostic interactionswere calculated by Brintzinger and co-workers using the extended Hu¨ckel method, which offers a qualitative but valuable chemical picture of the problem. Their analysis indicate that the Zr-H(R) interaction is rather absent in the olefin complex, due to an unfavorable antibonding overlap between the bonding Zr-CH3 and C-H(R) orbitals. As the reaction proceeds along the reaction path, a net stabilization due to the Zr-H(R) interaction arises close to the transition state.187 According to Janiak, the R-agostic stabilization becomes important through an increase in electron deficiency of the metal that switches from a formally 16e- Zr in the Cp2ZrCH3(C2H4)+ reactant to the formally 14e- Zr in the Cp2ZrC3H7+ product.188 Similar ideas were developed by Grubbs and Coates, who also made a nice relationship between the hyperconjugative stabilization by β-hydrogen atoms of substrate undergoing nucleophilic substitution reactions in organic chemistry and the agostic stabilization by R-hydrogen atoms in Ziegler-Natta catalysis (Figure 15).102 Another estimate of the transition state stabilization due to the R-agostic interaction can be obtained by looking at the plot reported in Figure 16. The continuous line represents the DFT energy of the transition state for propene insertion into the Zr-

Figure 17. Stereokinetic isotope effects in the polymerization of E- and Z-propene-1-d1 using a C2-symmetric metallocene. The transition state with an R-hydrogen agostic interaction is considered to be more stable than the transition state with an R-deuterium agostic interaction. Reprinted from ref 102. Copyright 1996 American Chemical Society.

methyl bond of the H2Si(Cp)2ZrCH3+ system, as a function of the Zr-H(R) distance. The fully optimized metallocene-propene complex is assumed as the reference state at 0 kcal/mol. It is clear that the transition state energy is sensibly dependent on the Zr-H(R) distance and that the substantially low energy barrierscorresponding to the energy difference between the minimum of the two curves, 5.0 kcal/molswould be quite higher if the Zr-H(R) distance in the transition state would be fixed at 3.0 Å, which is the value that the Zr-H(R) distance assumes in the olefin complex. However, it has to be noted that at too high values of the Zr-H(R) distance, the methyl group cannot tilt away from the Zr-C axis and point the sp3 orbital in an optimal way to enhance the incipient bonding interactions with the olefin. This represents a further contribution to the transition state destabilization at high Zr-H(R) distances. For ethene insertion with the neutral d0 scandocene Cp2ScCH3,152,174 the R-agostic interaction is less pronounced. The weaker R-agostic interaction was ascribed to the less electron deficiency of Sc with respect to Zr and was argued to be at the origin of the slightly higher insertion barrier, about 3 kcal/ mol.152,174 The relevance of R-agostic interactions has been experimentally investigated by using isotopically labeled substrates to probe for their role during olefin insertion.193,194 Following a reasoning developed by Grubbs et al.,102,195 Kraudelat and Brintzinger investigated the hydrodimerization of deuterated 1-hexene with Cp2ZrCl2/MAO.193 They measured erythro/threo ratios in accordance with a value of kH/kD ≈ 1.3,187 which is consistent with an R-agostic assisted insertion reaction.102,195 Moreover, Leclerc and Brintzinger performed polymerization of E- and Z-propene-1-d1 using a C2-symmetric metallocene.194,196 As shown in Figure 17, polypropenes made from the E-isomer should have molecular weights greater than polymers


made from the Z-isomer, if the insertion reaction is R-agostic assisted, since the E-isomer should correspond a faster insertion rate. This holds in the assumption that chain release reactions rates are equal for both isomers. These elegant mechanistic studies gave as a result that polymers made from the E-isomer had molecular weights about 1.3 times greater than polymers made from the Z-isomer, indeed, again in accordance to a value of kH/kD ≈ 1.3.187 Finally, similar results were obtained with neutral scandocene catalysts by Piers and Bercaw.197 They observed similar kinetic isotope effects in the hydrocyclizations and hydrodimerizations of deuterated species. Before concluding this section, it has to be noted that the presence of an R-agostic interaction facilitates the insertion reaction, but it is not necessary. As pointed out by Brintzinger and co-workers, whenever the electron deficiency of the transition state is diminished, by electron-donating ligands, coordination of a solvent molecule or of a second olefin, or some contact with the counterion, the agostic stabilization would undoubtedly lose most of its advantage.187 Moreover, the substantial enantioselectivity of propene insertion into a secondary polypropylenic growing chain on C2-symmetric metallocenes,198,199 as after a regioirregular insertion, indicates that in this case the insertion reaction occurs with a methyl group occupying the position usually taken from the R-agostic hydrogen atom. This idea finds support in the molecular mechanics study of Corradini and coworkers.200 However, Brintzinger also noticed that the presence of an R-agostic interaction at the transition state is entirely compatible with the steric requirements of substituted chiral ansa-metallocenes.187 That is, the preferred transition state geometry based on electronic considerations is remarkably similar to the transition state geometry based on steric requirements (the so-called “chiral orientation of the growing chain” mechanism; see section III.H).

E. Insertion Barrier Regarding the height of the insertion barrier, the situation is much more controversial. Since simplified ligands as Cl or Hsoften used to model Cp ringsssubstantially increase the height of the insertion barrier, we limit this discussion to calculations including full Cp rings. The first prediction of the barrier for the insertion reaction Cp2TiCH3+ + ethene by Jolly and Marinick gave a barrier of 9.8 kcal/mol at the MP2 level.170 However, it has to be considered that their geometries were determined by using a simpler semiempirical method, and only energetics were evaluated at the MP2 level. The same insertion reaction was studied by Ahlrics and co-workers, which found a considerable energy barrier and a transition state only without inclusion of electron correlation.149 On a correlated level, they found that the insertion reaction occurs on a very flat, downhill potential energy surface. Similar conclusions were also reached by Meier and co-workers, who investigated the ethene + H2Si(Cp)2TiCH3+ reaction by using the Car-Parrinello method.156 For the ethene


insertion reaction on the Cp2ZrR+ (R ) CH3, C2H5) and H2Si(Cp)2ZrCH3+ zirconocenes and on the Cp2ScCH3 scandocene, the static DFT calculations of Ziegler and co-workers predicted almost negligible insertion barriers, Ph2Si > Me2Si, while molecular weights decrease in the order Ph2C > PhP > CH2CH2 > Me2C ∼ Me2Si > Ph2Si. The effect of the metal is quite analogous to the case of C2-symmetric zirconocenes: Hf is less active and produces higher molecular weights compared to Zr, while Ti is much less active and also much less enantioselective compared to the other two metals; indeed, both Ph2C(Cp)(9-Flu)TiCl2/MAO and 1,1′,2,2′(Me2Si)2(4-i-PrCp)(3,5-i-Pr2Cp)TiCl2/MAO produce fully atactic PP ([rr] ) 0.23 at -60 °C).416 The probability of back-skip can be calculated by the method described by Farina,409 provided the whole set of pentads is known. The available values are shown in Table 10. An ambiguity remains as to the source of rrmr stereoerrors, since these can arise from a skipped insertion between two stereoregular insertions or by a chain back-skip after a stereoerror (Scheme 28).113 If this were the case, then the rmmr pentad would not be a good indicator of the enantioface stereoerrors, since a part of these errors would be “erased” by the following back-skip and turned into a rmrr pentad (Scheme 28). A stereoirregular insertion could even favor back-skip. The methyl pentad region of two s-PP samples of different stereoregularities are compared in Figure 34. The dependence on polymerization temperature has been studied for Cs-1,113,417 Cs-5,364 Cs-8,46 and Cs14 (and related systems).412 For the latter system, the increase of catalyst activity and corresponding decrease in molecular weight with increasing Tp is quite remarkable. The detrimental influence of decreasing propene concentration113,412 and of increasing amounts of CH2Cl2 solvent417 on syndiospecificity is in line with unimolecular chain back-skip (also called site epimer-

Figure 34. Methyl region of the 13C NMR spectra of two s-PP samples showing different levels of rrmr (site isomerization or back-skip of the chain) and rmmr stereoerrors; top, prevailingly enantioface errors; bottom, increased site isomerization.

ization) mechanism competing with bimolecular propagation. However, the nonlinear activity versus [M] correlation for the syndiospecific Cs-1 catalyst113 argues against a simple bimolecular propagation scheme418 (see section VIII for details). For Cs-5, one can estimate a ∆∆E‡enant ) 2.1 ( 0.1 kcal/mol, in the rough estimate that the rmmr pentad accounts for all enantioface insertion errors.364 This value is similar to that found for the least isospecific C2symmetric zirconocenes (see section V).

C. C1-Symmetric Metallocenes: from Hemiisotactic to Isotactic Polypropene C1-Symmetric metallocenes are, broadly speaking, complexes lacking any symmetry element. Of course, such a broad definition includes a very large number of possible structures. For the purpose of this review, there are two types of C1-symmetric metallocenes which are of interest, all of them bridged, hence stereorigid: those with one (substituted or nonsubstituted) cyclopentadienyl ligand having two homotopic faces (e.g. C1-I-1 in Chart 19) and those having two asymmetric cyclopentadienyls (e.g. C1-II-1 in Chart 19). The first type, C1-I, presents a synthetic advantage with respect to the second type, and also with respect to the isospecific C2-symmetric complexes. In fact, a problem associated with the synthesis of ansa-C2-symmetric metallocenes is that they are almost invariably generated along with their meso isomers (Scheme 29), which are difficult to remove from the catalyst mixture and often produce unwanted low molecular weight atactic polypropene with nonnegligible polymerization activity. In addition, most of these modified C2-symmetric systems require multistep, low overall yield synthetic routes.320,336 On the contrary, for C1-symmetric systems of type I, a meso form does not exist (Scheme 29B). In several cases, the synthesis of the ligand is also quite simple, as in the Me2C(3-R-Cp)(9-Flu)ZrCl2 complexes shown in Scheme 29B. The common feature of C1-symmetric metallocenes is that their two coordination sites are diastereotopic. Because of this property, and depending on the size of the substituent on the cyclopentadienyl ligand, C1-


Resconi et al.

Chart 19. Representative C1-Symmetric Metallocenes

symmetric catalysts can vary in stereoselectivity from hemiisospecific (producing hi-PP, which is amorphous) to partially isospecific (producing amorphous or low crystallinity PP, materials that are thermoplastic-elastomeric in nature) to isospecific (producing crystalline i-PP). As in the case of C2-symmetric systems, all degrees of isotacticity and molecular weights can be obtained, as the possible ligand variations are even broader. Some representative metallocenes of the two types are shown in Chart 19. The simplest C1-symmetric complex, C1-I-1, its analogues with Hf or Ti, and its Me2Si-bridged congener

have been extensively investigated, but their catalytic performances are inversely proportional to the number of studies in which they have appeared.390-393,395,420-423 The same low catalytic activity and low polypropene molecular weight have been observed for erythro/threo-Me2C(3-MeCp)(1-Ind)ZrCl2424 and for Chisso’s biscyclopentadienyl systems.310 Going to bulkier ligands improves PP molecular weights. We start by analyzing in detail the case of the hemiisospecific complexes, the prototype of which is C1-I-6.


Hemiisospecific C1-Symmetric Metallocenes. The first effectively hemiisospecific metallocene catalyst, Me2C(3-MeCp)(9-Flu)2ZrCl2 (C1-I-6), and its Hf analogue, which are chiral and lacking any symmetry, and the polypropene produced therefrom have been reported by the Hoechst and Fina groups.46,421,425,426 Ewen correctly assigned the stereochemical distribution of the chirotopic methines in this peculiar microstructure as hemiisotactic, a microstructure previously assigned by Farina on a polymer synthesized indirectly.427 The catalytic synthesis of hemiisotactic polypropene, although this polymer remains a scientific curiosity, has been rather important in confirming the polymerization mechanism of metallocenes, since it could not be explained without the site-switching mechanism, as it is a case of syndiospecific polymerization. The presence on the same metal center of one isospecific and one aspecific site, together with the requirement of chain migratory insertion with site switching at (almost) every insertion, generates a unique polymer structure, in which every other methyne is in the same (isotactic) configuration, while the remaining alternating methynes are in a random configuration.46,427,428 Occasional back-skip of the chain accounts for the slight deviation from perfect hemiisotacticity, which requires a well-defined pentad distribution (3:2:1:4:0:0:3:2:1). Other metallocenes


with the proper ligand symmetry for hemiisospecific polymerization have been reported,385-395,422,429 but most of these catalysts produced low molecular weight PP. The highest molecular weights (M h w ∼ 200 000-300 000) are obtained with the hafnium analogue of C1-I-6, while the zirconium complex gives lower molecular weights (M hw ∼ 50 000). Both catalysts have a quite low activity.425 No physicomechanical characterization of this polymer has been reported so far, but it is expected to have some elastomeric properties. In some cases the elastomeric properties of the polypropenes have been confirmed. As outlined above, for C1 symmetric catalysts the two available coordination positions are nonequivalent (diastereotopic).410 In this case, the stereoselectivity of models of the catalytic system depends on the energy difference between structures corresponding to propene coordination on the two nonequivalent inward and outward coordination positions, and two general cases can be considered.430 The Two Coordination Positions Are of Similar Energy. In the hypothesis that the chain migratory insertion mechanism is still prevailing, the model of these catalytic systems would be isospecific or syndiospecific, if the two situations originated from propene coordination on the two coordination positions are enantioselective in favor of the same or opposite propene enantiofaces, respectively. If only one situation is enantioselective, the corresponding catalytic system is hemiisospecific. The Two Coordination Positions Are of Substantially Different Energy. For models of these catalytic systems, the sequence of chain migratory insertion steps can be altered. In fact, the growing chain in successive coordination and insertion steps can often occupy the same coordination position. That is, after each migratory insertion step, in the absence of a coordinated monomer molecule, the growing chain could swing back to the previous coordination position (back-skip of the growing chain). The driving force for the back-skip of the chain could be the energy difference between the two diastereomeric situations obtained by exchanging the relative positions of monomer and chain. Of course, the probability of occurrence of a back-skip of the chain, in the alkene-free state, is only indirectly dependent on this energy difference. In fact, it is dependent on the difference between the activation energy for the chain back-skip, E‡back-skip, and the activation energy for the formation of the high energy alkene-bonded intermediate, E‡coord,out (see Scheme 30). However, since the degree to which empirical force fields can be used for prediction of transition states is not well-established and since the activation energy E‡coord,out is expected to increase with increasing Eout, for the sake of simplicity, we take Eout - Einw as a semiquantitative evaluation of the driving force for the back-skip of the chain. The models of hemiisospecific catalytic systems generally correspond to C1-symmetric systems for which the two coordination positions are of similar energy. The best known example is the model based on the Me2C(3-MeCp)(9-Flu) ligand (entry 34 in


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Scheme 30

Chart 8 and Table 3), independently investigated by Corradini and Fink.89,90 The models of Figure 35 report the minimum energy situations corresponding to propene coordination to the catalytic system based on the aforementioned ligand. Clearly, the model is enantioselective when the propene molecule is inward coordinated (which corresponds to a R chirality at the metal atom for a (R) coordination of the 3-MeCp ligand), since structure A, with a re-coordinated propene, is favored relative to structure B, with a si-coordinated propene, in the framework of the mechanism of the chiral orientation of the growing chain. In fact, structure B is disfavored by repulsive interactions between the growing chain and the fluorenyl ligand. On the contrary, the model is nonenantioselective when the growing chain is σ-bonded in the inward coordination position (which correspond to a S chirality at the metal atom for a (R) coordination of the 3-MeCp ligand) since structures C and D are equally hampered by repulsive interactions of the growing chain either with the 9-Flu ligand (structure C) or with the methyl group of the 3-MeCp ligand (structure D). Moreover, it can be reasonably assumed that the polymerization prevailingly occurs according to a regular chain migratory insertion mechanism, since the situation corresponding to inward propene coordination is only slightly favored with respect to situations with outward propene coordination.89,90 Hence, every each other insertion is enantioselective, while the other is nonenantioselective. Consequently, the model is hemiisospecific. This analysis is in qualitative agreement with the experimentally observed production of hemiisotactic polymer, with catalytic systems based on the bridged (3-MeCp)(9-Flu) ligand.46,118,421 Furthermore, Fink systematically investigated the effect of the alkyl group R in position 3, by considering also systems with R ) ethyl and isopropyl. Both systems have been calculated to have one poorly enantioselective geometry (when the growing chain is inward coordinated) and one nicely enantioselective geometry (when the growing chain is outward coordinated). Since enantioselective geometries are slightly lower in energy than nonenantioselective

Figure 35. Molecular mechanics minimum energy geometry for re and si propene coordination on the Me2Si(3-MeCp)(9-Flu)Zr(isobutyl) model with a R and S chirality at the metal atom, parts A and B, and C and D, respectively. (R) is the chirality of coordination of the 3-Me-Cp ligand.

geometries (less than 1.5 kcal/mol), the corresponding catalytic systems should be hemiisospecific. This prediction89 has been confirmed experimentally only in general terms, by polymerizations catalyzed with C1-I-7 and C1-I-8431 (see below). For the C1 symmetric ligand Me2C(Cp)(1-Ind), similar calculations have shown that the geometry with a propene molecule inward coordinated is nonenantioselective, while a weak enantioselectivity is calculated when the propene molecule is outward coordinated.89,390,423 This is in qualitative agreement with the experimentally observed production of prevailingly atactic polymer and of hemiisotactic polymer at low temperatures, with catalytic systems based on C1-I-1 and related systems, although the low PP molecular weights hamper a precise methyl pentad analysis, because of extensive overlapping with resonances due to end groups.390-393,395,420-423 Isospecific C1-Symmetric Metallocenes. Perfect hemiisotacticity requires that mmmm ) 18.75%. Deviating from the structure of C1-I-6 in general makes the complexes more isospecific. Fink studied the series C1-I-6-C1-I-9 where the R substituent on Cp increases from methyl to tert-butyl. While the ethyl derivative produces a polypropene very similar to that (prevailingly hemiisotactic) made with the methyl derivative C1-I-6, the 3-i-PrCp derivative C1I-8 431 and its Me2Si-bridged analogue341 produce PP with mmmm of 44% (Tp ) 70 °C) and 64.4% (Tp ) 60 °C) respectively, compared to 14-27% with C1-I-6.431 While C1-I-11 produces a hemiisotactic PP (mmmm ) 9.9-12.0% on going from Tp ) 10 to Tp ) 70 °C at 2 bar propene),431 Me2Si(1-Ind)(9-Flu)ZrCl2 produces (at Tp ) 50 °C) PP with mmmm ∼ 57%.432 On going


to the 3-t-BuCp, the catalyst isospecificity (within this class) is maximized:433 C1-I-9 produces i-PP with relatively high mmmm pentad contents from 77.5% (Tp ) 60 °C in liquid monomer 364) to 87.8%. (Tp ) 50 °C in toluene at 2 bar propene 431). Its Me2Si-bridged congener is even more isospecific, producing i-PP with higher melting points (e.g. Tm ) 161 °C vs 130 °C with C1-I-9, for Tp ) 30 °C).433 Similar results have been reported by Marks with C1-I-5 (mmmm ) 35% at Tp ) 25 °C) and its Hf analogue (mmmm ) 60-83% at Tp ) 25 °Csquite remarkable is the higher isospecificity of Hf vs Zr)434 and C1-I-16 (mmmm ) 83% at Tp ) 25 °C).435 Compared to the best C2-symmetric catalysts, C1I-5 and related systems have lower stereoselectivity and produce i-PP with modest molecular weights. Possibly because of the presence of one highly hindered polymerization site, their polymerization activities and molecular weights seem in general lower than those of the more isospecific C2-symmetric systems. Along the same line, Miyake and co-workers have developed a class of C1-symmetric systems which are highly isospecific (e.g. threo-Me2C(3-t-Bu-Cp)(3-t-Bu1-Ind)ZrCl2, C1-II-2a) but again produce low molecular weight i-PP. In this case, the meso-like (erythro) isomer (C1-II-2b, less active than the racemic one) is partially isospecific.323 C1-II-2a is remarkably insensitive to the polymerization temperature in terms of isospecificity,323,339 while molecular weights drop from 100 000 to 9 000 on going from 1 to 60 °C.323 The Ti analogue of C1-II-2a offers a rare example of a titanocene being more active than the corresponding zirconocene, while the same very high isospecificity is maintained. The Hf analogue obeys the general rule, showing a 20-fold decrease in polymerization activity compared to C1-II-2a. The silicon-bridged C1II-3 is less active and less stereoselective, and produces lower molecular weights than C1-II-2a. Both C1-II-2a and C1-I-9323,364 produce i-PP containing minor amounts of regioirregularities, being more regioselective than most C2-symmetric catalysts. The influence of [M] (propene concentration) on the isospecificity of C1-II-2a is low compared to the C2symmetric systems (see section V). Interestingly, the isospecificity of C1-I-9 also seems to be independent of Tp.364 However, Fink has reported an increase of isotacticity from 83.5 to 87.8% mmmm on going from Tp ) 10 to 50 °C, operating at constant propene pressure of 2 bar, rather than at constant [M].431 Hence this effect can be ascribed to a lower propene concentration at the higher temperature, which increases the chance of unimolecular chain back-skip over insertion. The best performance of a C1-symmetric system so far has been attained by Spaleck and co-workers with the zirconocenes C1-II-4 and C1-II-5,436 which combine the indenyl substitution of two different C2-symmetric zirconocenes previously described by the Hoechst group, thus providing both high stereoregularity and high molecular weights (for example, C1-II-5 gives i-PP with mm ) 96%, 2,1 ) 0.4%, Tm ) 155 °C and M h w ) 530 000, at the relatively high polymerization temperature of 70 °C).


Figure 36. Molecular mechanics minimum energy geometry for re and si propene coordination on the Me2Si(3-tBu-Cp)(9-Flu)Zr(isobutyl) model with a R and S chirality at the metal atom, parts A and B, and C and D, respectively. (R) is the chirality of coordination of the 3-t-Bu-Cp ligand.

All these aspects can be fully rationalized by molecular modeling, by the usual nonbonded interaction analysis. From a molecular modeling standpoint, Morokuma,275 Fink,89 and Corradini273 have confirmed that the size of the substituent in position 3 has a remarkable relevance also for these C1 symmetric catalysts based on the H2Si(Cp)(9-Flu)275 and Me2C(Cp)(9-Flu)89,273 ligands, the latter corresponding to entry 30 in Chart 8. All authors agree that replacing hydrogen with a tert-butyl group in position 3 of the Cp ring (see Figure 36) turns the catalyst from syndiospecific to isospecific. However, some subtle differences do exist between the different calculations. According to Morokuma, the presence of the tert-butyl group forbids the growing chain to be located in the inward position (i.e. close to the tertbutyl group). In particular, in the absence of the monomer molecule, as probably occurs at the end of each insertion step, the steric pressure of the ligand skeleton could force the growing chain to skip back to the less crowded outward position. Hence, insertion always occurs with the same relative disposition of the monomer and of the growing chain (inward and outward, respectively), and the model is consequently isospecific. On the contrary, Fink and Corradini have found that insertion can occur with the growing chain in the crowded inward position as well, since the growing outward chain is favored relative to the growing inward chain by roughly 1-4 kcal/mol.89,273 However, both geometries (corresponding to inward and outward growing chain coordination) favor the same propene enantioface (in the models of Figure 36, insertion of the re enantioface is favored for both inward and outward propene coordinations). Hence, also the models of Fink89 and Corradini273 are able to rationalize the experimental isospecific behavior of this catalyst.433 Probably, the frequence of backskip of the growing chain in the less crowded outward


position is considerably high for this catalyst, although consequences on stereoselectivity are scarce, since whatever the coordination position of the growing chain, enantioselectivity is reasonably high and in the favor of the same propene enantioface. It is clear that the most important aspect of C1symmetric zirconocenes is the wide variability of their isospecificity, which has allowed the preparation of novel PP materials. This avenue has been opened by Chien and Rausch, who reported the preparation of thermoplastic-elastomeric polypropene (TPE-PP) with C1-I-3-anti and its more active isomer C1-I-3-syn.385-388,437 The Zr analogue of C1I-3 is practically inactive.389 It is worth noting here that the TPE-PP produced with the Chien/Rausch catalysts is markedly different from the TPE-PP made with Waymouth’s catalysts. In fact, the former (mmmm ∼ 40%) has a very low crystallinity (Tm ∼ 50-65 °C, ∆Hf ∼ 3 cal/g after annealing) and is an elastomeric polymer fully soluble in Et2O,437 while TPE-PP from (2-Ar-Ind)2ZrCl2 catalysts can be separated in fractions of widely different crystallinity, although the average methyl pentad content is quite similar in the two cases (see section IV.A.3). Following this path, several examples of C1-symmetric zirconocenes which produce elastomeric polypropenes of different tacticity have been produced. Rieger prepared the C1-symmetric zirconocenes rac-C2H4(9-Flu)(1-Ind)ZrCl2 and the two diastereoisomers of rac-C2H3-1-(R,S)Ph-1-(9-Flu)-2-((R,S)-1-Ind)ZrCl2 (C1-I-13) and of rac-C2H3-1-(R,S)Ph-(Cp)((R,S)-1-Ind)ZrCl2 (C1-I-4).438 In particular, rac-C2H31-(R)Ph-1-(9-Flu)-2-((R)-1-Ind)ZrCl2 isthemostisospecific. Interestingly, this catalyst produces a low melting PP (Tm ) 98-121 °C), although molecular weights are too low for any practical use. The latter have a low enantioselectivity. Me2Si(Cp)(2-p-tolyl-1-Ind)ZrCl2 produces (in toluene at 1.2 atm propene and 25 °C) an atactic polypropene (mm ) 31%) of remarkably high molecular weight (Mv ) 377 000).439 Low-pressure propene polymerization with antiMe2Si(1-Ind)(3-Me-1-Ind)ZrCl2 produces elastomeric PP with mmmm contents of 30-50% depending on propene concentration and polymerization temperature. 394 C1-I-12 and related systems produce PP with mmmm pentad contents ranging from 7 to 80% but of very low molecular weights. 440 C1-I-14 and C1-I15 again produce PP with mmmm ranging from 54 to 80% but with higher molecular weights. 295 The large difference in molecular weights between C1-I12 and the sterically very similar C1-I-15 is quite surprising. High molecular weights have also been reported with C1-I-17 which produces (30 °C, toluene, [M] ) 1.29 mol/L) PP with mmmm ) 57.5%.441 Corradini and co-workers modeled the catalysts based on the C2H3[1-(9-Flu)-1-Ph-2-(1-Ind)] and on the C2H3[1-Cp-1-Ph-2-(1-Ind)] ligands presenting R and S chirality, at the C(1) atom of the ethylene bridge (entries 36 and 38 and entries 37 and 39 in Chart 8).92 The R and S configurations at the C(1) atom of the bridge favor the δ- and λ-bridge conformations, respectively.438,442,443 For the complex containing the C2H3[1-(9-Flu)-1-Ph-2-(1-Ind)] ligand with

Resconi et al.

δ-bridge conformation (entry 36 in Table 3), the situations corresponding to outward and inward propene coordination are partially and strongly enantioselective, respectively, in favor of the same enantioface, while for the same ligand but with λ-bridge conformation (entry 37 in Table 3), the situations corresponding to outward and inward propene coordination are partially and strongly enantioselective in favor of opposite enantiofaces. Hence, the model with δ-bridge conformation tends to be isospecific, whereas the model with λ-bridge conformation is substantially hemiisopecific. Analogously, for the complex containing the C2H3[1-Cp-1-Ph-2-(1-Ind)] ligand, the model with δ-bridge conformation has a slight tendency to be isospecific with respect to the model with λ-bridge conformation (entries 37 and 39 in Table 3, respectively). Moreover, for the complex containing the C2H3[1-(9-Flu)-1-Ph-2-(1-Ind)] ligand, the more enantioselective geometry is favored by more than 2 kcal/mol over the poorly enantioselective geometry, independently of the bridge conformation. Hence, at low monomer concentration a more frequent back-skip of the chain toward the energetically favored and more enantioselective geometry is reasonable. This could explain the experimentally observed increase of stereospecificity at low monomer concentration.438 Differently, for the complex containing the C2H3[1-Cp-1-Ph-2-(1-Ind)] ligand, the more enantioselective geometry is only slightly favored (roughly 0.5 kcal/mol) over the poorly enantioselective geometry, independently of the bridge conformation. Hence, a driving force for a frequent back-skip of the chain toward the more enantioselective geometry is absent. This could explain the substantial independence of the stereospecificity from monomer concentration experimentally observed.438 The energy differences between minima corresponding to diastereomeric preinsertion intermediates with different chiralities at the central metal atom (Eout - Einw) for several catalytic models with C1-symmetric metallocenes are listed in the last column of Table 3. It is worth noting that, when substantial energy differences between minimum energy diastereomeric intermediates are present, lower energies correspond to the monomer coordination in the (more crowded) inward coordination position. In particular, it is reasonable to expect that, for models with large Eout - Einw values (34-37), the growing chain in successive coordination steps can occupy frequently the (less crowded) outward coordination position, leaving the inward position free for the monomer coordination. For all these models, the lower energy diastereomer with inward monomer coordination is more enantioselective than the higher energy diastereomer with outward monomer coordination. As a concluding remark, C1-symmetric metallocenes have been quite valuable in at least three aspects: (i) increasing the range of achievable microstructures and indeed giving access to a range of thermoplastic-elastomeric, homogeneous polypropenes; (ii) increasing the complexity of the polymerization mechanism, hence providing new stimuli which in turn gave a deeper understanding of the


correlation between ligand structure and polymerization conditions on one side, and kinetics of insertion/site isomerization on the other; and (iii) providing models for the MgCl2/TiCl4 heterogeneous catalysts which proved better than those based on C2-symmetric active centers.444

Chemical Reviews, 2000, Vol. 100, No. 4 1309 Scheme 31. Kinetic Scheme for Epimerization. Keq ) [CM]/([C][M]) (Modified from ref 232)

V. Stereocontrol: Influence of Polymerization Conditions The catalytic performance of metallocene catalysts depends strongly on the polymerization conditions, much more so than, for example, in the case of MgCl2supported TiCl4 catalysts. The crucial importance of this behavior has been overlooked for many years. In fact, until recently, low-pressure propene polymerizations in toluene were the typical way metallocene catalysts were investigated. In these experiments, propene concentrations vary dramatically, especially when different polymerization temperatures are compared or high productivities are involved (because of diffusion-limited monomer concentrations). As a consequence, literature data on polymerization activity and degree of regio- and stereoselectivity of the prototypical isospecific metallocene catalysts varied in a wide range and made comparisons difficult. Although differences in catalyst activities could be readily explained in terms of the different experimental conditions such as handling procedures, source of the MAO cocatalyst, and purity of the metallocene precatalyst, the scatter of polymer regioregularity, stereoregularity, molecular weight, solubility, and melting point data could not be explained by the usual differences among different laboratories. The first studies on the dependence of catalyst performance on propene concentration with zirconocene catalysts have been reported by Ewen, for the syndiospecific catalyst Cs-1,113 and by Rieger, for a series of C1-symmetric zirconocenes.438 The influence of monomer concentration on the behavior of isospecific C2-symmetric metallocenes has been neglected for a long time, with the exception of an early work,305 which, however, was limited in its scope to propagation rate and molecular weight aspects with the system C2-I-1/MAO in a relatively narrow monomer concentration range, until at the STEPOL meeting in 1994, two groups independently reported on the dramatic influence of propene concentration, [M], on the stereoselectivity of C2-symmetric zirconocene catalysts.445,446 While providing a much needed explanation for the observed inconsistency of experimental data, this finding has washed away a lot of previous experimental work. In the following two sections, we discuss the influence of monomer concentration first, and then the influence of polymerization temperature on those investigations only that have taken into account the influence of [M].

A. Influence of Monomer Concentration 1. C2-Symmetric Catalysts Several C2-symmetric ansa-zirconocenes have been studied to determine the extent of the influence of monomer concentration on catalyst activity, i-PP stereoregularity, and molecular weight. Brintzinger

showed that the molecular weight dependence on monomer concentration can be explained by the competition between monomolecular (β-hydrogen transfer to the metal) and bimolecular (β-hydrogen transfer to the monomer) chain release reactions and that the ratio between the two reaction rates strongly depends on the type of cyclopentadienyl ligand.336 The increase of i-PP molecular weight with propene concentration had been reported by Kaminsky, who explored the range 0.6-5 mol/L at 35 °C with C2-I1/MAO305 and then obtained propene oligomers with a related catalyst at 50 °C and very low monomer concentration.127 While the effect of [M] on molecular weight and type of regioerrors was to be expected (and actually prompted our original study of this variable), the detrimental influence of [M] on isotacticity was totally unexpected. For both benchmark catalysts C2-I-1 and C2-I-1H4, isotacticity (% mmmm) decreases substantially, e.g. from 87 to 54.7% by decreasing the monomer concentration from 11 to 0.4 mol/L in the case of C2-I-1. Under “catalyst starvation” conditions, that is [M] f 0, the resulting oligomers were fully atactic.232 On the basis of what we have discussed above, as far as enantioface selectivity is concerned, C2-symmetric chiral metallocenes should not sense on what side, and how often, a monomer approaches the metal center. Indeed, the site control mechanism of these catalysts requires that the stereochemistry of insertion is independent from the previous insertion. The loss of stereospecificity with the decrease in monomer concentration has been accounted for, from a kinetic standpoint, with an equilibrium between active sites having a coordinated monomer (C‚M) and sites without coordinated monomer (C) (Scheme 31). It is apparent from Scheme 31 that C, not having a coordinated monomer molecule, cannot generate m, r diads by monomer insertion. As a consequence, C must be able to racemize the chiral carbon of the last inserted unit, that is, as also pointed out by Busico and Cipullo and experimentally proven by Brintzinger and Leclerc by polymerizing Z- and E-propene1-d,194,196 C is an epimerization catalyst. In other words, the loss of isospecificity with decreasing [M] must be caused by a unimolecular process. The mechanisms proposed for this epimerization reaction are discussed in section V.C. In Scheme 31, pe is the probability of epimerization (racemization) of the last stereogenic methine, b is the probability of a correct enantioface insertion (re at a (R,R) center and si at a (S,S) center in the case of C2-I-1 and related catalysts), and the sites C and C‚M are related by

[C‚M] ) [C]Keq[M]


Resconi et al.

>99.5% at [M] g 2 mol/L down to 95.6% at [M] ) 0.2 mol/L.447

B. Influence of Polymerization Temperature

Figure 37. Experimental bobs/(1 - bobs) values: (9) C2-I35/MAO (50 °C, propene/pentane); (]) C2-I-1/MAO (50 °C, propene/toluene); solid line: fitting to eq 1. From ref 230.

Hence, assuming pe ) 1 - pe ) 0.5 (i.e. assuming C to lose any enantioface selectivity in the absence of coordinated monomer, as experimentally observed in the case of [M] f 0), eq 1 is obtained:

bobs 0.5 + bKeq[M] ) 1 - bobs 0.5 + (1 - b)Keq[M]

(1)

where bobs and (1 - bobs) (obtained through a leastsquares method from the methyl pentad distribution of i-PP) represent the observed concentration of correct and wrong primary insertions, respectively, at a given [M] and Tp, and the Bernoullian probability parameter b is the inherent enantioface selectivity which depends on the catalyst structure and Tp, but is independent from [M]. For C2-I-1/MAO and related catalysts, in liquid monomer at Tp below 70 °C, bobs f b, while the plateau regime is not reached for higher temperatures or for the bulkier 3-tBu-indenyl system C2-I-35. The consequence of the above results is that the active center needs a coordinated monomer molecule in order to retain its stereoselectivity. Also the stereoselectivity of both C1-symmetric (see sections IV.C and V.A.2 below) and unbridged catalysts appears to have a relevant dependence on monomer concentration, but for different reasons.366,431 The experimental bobs/(1 - bobs) values, fitted to eq 1, for C2-I-1/MAO and C2-I-35/MAO are shown in Figure 37.

2. C1-Symmetric Catalysts C1-symmetric zirconocenes behave rather differently. Rieger found an inverse dependence of isotacticity on monomer concentration for the C1-symmetric zirconocenes rac-C2H4(9-Flu)(1-Ind)ZrCl2 and the two diastereoisomers of rac-1-(R,S)Ph-C2H3-1-(9-Flu)-2((R,S)-1-Ind)ZrCl2, while the related rac-1-(R,S)PhC2H3(Cp)((R,S)-1-Ind)ZrCl2 has a low enantioselectivity, independent of monomer concentration.438 In particular, rac-C2H3-1-(R)Ph-1-(9-Flu)-2-(1(R)-Ind)ZrCl2 is the most isospecific, and mmmm increases from 46 to 80% on lowering [M] from 3.38 to 0.45 mol/ L. The most likely explanation of this behavior is the back-skip of the chain (see section IV.C). The highly isospecific C1-symmetric threo-Me2C(3t-Bu-Cp)(3-t-Bu-1-Ind)ZrCl2/MAO suffers only a minor decrease of isospecificity at 40 °C in toluene by lowering [M], producing i-PP with mmmm from

Another most important source of variability in the molecular architecture of polypropenes obtained from ansa-zirconocenes, besides the biscyclopentadienyl ligand structure and monomer concentration, is the polymerization temperature, Tp. Unfortunately, most of the earlier catalytic studies on the performance of metallocene catalysts have been carried out in solution at largely different propene concentrations, so changes in the latter due to lower propene concentrations at the higher Tp become the primary cause for changes on both polymer properties and polymerization kinetics, rather than Tp itself (see previous section). It is therefore of the utmost importance, when comparing the polymerization performance of different zirconocene catalysts, to perform the experiments under high and identical monomer concentrations, and preferably in liquid propene, to minimize the extent of chain-end epimerization. Several C2-I-type zirconocenes have been investigated. Both isotacticity and molecular weight of i-PP decrease by increasing Tp, while the amount of secondary insertions, when present, increases slightly. For example, upon increasing Tp from 20 to 70 °C, C2-I-1/MAO yields polypropenes with decreasing M hv values from 56 000 to 19 600, percent mmmm pentads from 92 to 83%, and corresponding melting temperatures from 142 to 125 °C. Furthermore, the overall fraction of regioirregularities (2,1 and 3,1 insertions) increases from 0.4 to 0.7%. C2-I-9, which is slightly less isospecific than C2-I-1/MAO due to a wider “bite angle” β (see Table 1), produces i-PP with percent mmmm pentads decreasing from 88 to 77% by increasing Tp from 20 to 70 °C. The regioselectivity of C2-I-9 is slightly higher (percent 2,1 insertions from 0.14 at 0 °C to 0.60% at 70 °C) than that of C2-I-1. It is worth noting here that there is no detectable 2,1 f 3,1 isomerization with C2-I-9/MAO, as only 2,1erythro and 2,1-threo units were observed (see section VII.A for details). As the 2,1 f 3,1 isomerization reaction would be faster (relative to the following primary insertion) than epimerization, its absence (or very low extent, as in C2-I-1/MAO) is an indication of the absence of epimerization in liquid monomer. Molecular weights of i-PP from C2-I-9/MAO are lower than those obtained from C2-I-1/MAO at any temperature in the range investigated, ranging (ash n ≈ 2) from M h v ≈ 20 000 (M h n ) 11 000 suming M h v/M at Tp ) 0 °C) to M h v ≈ 12 000 (M h n ) 6000 at Tp ) 70 °C). It is also interesting that the temperature dependence of molecular weights of i-PP from C2-I9/MAO is lower than that shown by C2-I-1/MAO (see below). Ewen has reported that rac-C2H4(3-Me-1-Ind)2ZrCl2/MAO catalyst (C2-I-31/MAO) is nearly aspecific despite its C2-symmetry.116 We have reinvestigated the behavior of C2-I-31/MAO in liquid monomer and confirmed that it is far less isospecific than C2-I-1/ MAO: percent mmmm pentads decrease from 36 to 14% (b50 °C ) 0.7233) by increasing Tp from 0 to 70 °C.50



Scheme 32. Schematic Representation of the Origin of ∆∆E‡ in (R,R)-C2-I-1

The polymerization mechanism is not immediately obvious by looking at the pentad region of the 13C NMR, due to the presence of all 10 pentads. Application of the statistical triad tests allows one to identify the source of the weak enantioface selectivity in C2I-31/MAO as being enantiomorphic site control, as it is the case of all other C2-symmetric systems. This is possible by looking at the Tp dependence of the E and B triad tests (see section II.G): only the correct mechanism shows invariance with Tp of the corresponding triad test. The isospecific and highly regioselective C2-I-35/ MAO catalyst also shows a strong dependence toward Tp, producing i-PP with mmmm ranging from 97% at 20 °C to 91% at 70 °C and average viscosity molecular weights from 410 000 at 20 °C to 25 000 at 70 °C.50 The isospecificity of a catalyst is defined by the statistical parameter b, which represents the probability of a “correct” monomer insertion in the enantiomorphic site, at a given polymerization temperature. Assuming epimerization to be negligible in liquid monomer (bobs ) b), the Arrhenius plot of ln[b/(1 - b)] versus 1/Tp yields straight lines of slope ∆∆E‡/R, from which the values of enantioface selectivity ∆∆E‡enant ) |∆E‡si - ∆E‡re| is estimated (Scheme 32). A series of selected experimental ∆∆E‡enant values are compared in Table 11, while some of the ln[b/(1 - b)] versus 1/Tp plots for these systems are shown in Figure 38. The ∆∆E‡ values for C2-I-1 and C2-I-9 are intermediate between that of enantiomorphic site control of highly isospecific Ti catalysts (4.8 kcal/mol)114 and

Figure 38. Arrhenius plots of ln[b/(1 - b)] versus 1/Tp for selected C2-symmetric, bisindenyl zirconocenes. I: C2I-31/MAO; II: C2-I-9/MAO; III: C2-I-1/MAO; IV: C2-I-35/ MAO; V: C2-I-36/MAO. All polymerization in liquid propene.

that of chain-end control (ca. 2 kcal/mol).115,284 The lower isospecificity of C2-I-31 can be accounted for by the lesser steric difference between the facing methyl and benzene rings in the 3-methylindenyl moiety. The higher isospecificities of C2-I-35 and C2I-36 compared to C2-I-1 are remarkable, but ∆∆E‡ in the case of C2-I-35 is overestimated due to residual epimerization even in liquid monomer.229

C. Epimerization of the Primary Growing Chain As discussed in section V.A, in the case of propene polymerization with C2-symmetric zirconocenes, the isotacticity of PP decreases at lower propene concentrations, due to unimolecular primary-growing-chainend epimerization, which scrambles the chirality of the last chirotopic methine of the growing chain. The extent of epimerization at a given [M] depends strongly on the nature of the ansa-π-ligand252,446,448-450 and on the polymerization temperature.446 Epimerization has been explained by two mechanisms, both requiring formation of a Zr-H(CH2dCMeP) olefin complex via unimolecular β-H transfer. Busico’s mechanism (I in Scheme 33)448 involves a sequence of β-H transfers, double bond reorientations, and insertions. On the basis of literature precedents,144,225,253,255,451-457 Resconi has proposed that the reversible formation of a zirconocene allyl dihydrogen complex (II in Scheme 33) could be used to explain growing-chain epimerization.231,458 Formation of an allyl intermediate also accounts for the presence of

Table 11. Experimental and Calculated ∆∆E‡enant for Various C2-Symmetric Zirconocenes rac-zirconocene

∆∆E‡enant, kcal/mol, obsd

∆∆Eenant,a kcal/mol, calcd

∆∆E‡enant,b kcal/mol, calcd

C2H4(1-Ind)2ZrCl2 (C2-I-1) C2H4(3-Me-1-Ind)2ZrCl2 (C2-I-31) C2H4(4,7-Me2-1-Ind)2ZrCl2 (C2-I-18) Me2C(1-Ind)2ZrCl2 (C2-I-9) Me2C(3-Me3Si-1-Ind)2ZrCl2 (C2-I-33) Me2C(3-t-Bu-1-Ind)2ZrCl2 (C2-I-35) H2C(3-t-Bu-1-Ind)2ZrCl2 (C2-I-36)

3.3 ( 0.2 1.9 ( 0.2 3.1 ( 0.2 2.8 ( 0.2 2.6 ( 0.2 4.5 ( 0.5 3.7 ( 0.4

4.3 1.4 5.7 3.8 3.5 4.3

3.5 1.3 3.2 2.3 2.9 4.3

refc 50 50 202 50 50 50 324

a ∆∆E enant is the energy difference between the olefin complexes corresponding to the two propene enantiofaces, calculated by Corradini, Guerra, Cavallo, and co-workers according to the method described in refs 91 and 272. b ∆∆E‡enant is the energy difference between the approximated transition state geometries corresponding to the two propene enantiofaces, calculated by Corradini, Guerra, Cavallo, and co-workers according to the method described in refs 91 and 272. c The references reported in this column refer to the experimental ∆∆E‡enant values.


Resconi et al.

Scheme 33. Epimerization via Double Bond Reorientation (I) or Reversible Formation of a (R,R)Zr(allyl)(H2) Cation (II)a

a

The ligand bridge is omitted for clarity.

internal unsaturations131,229 and provides a model for reversible catalyst deactivation.225,453,454 Allyl rotation has been shown experimentally to be feasible in related systems.181,254 Both mechanisms take into account the analysis of Leclerc and Brintzinger,194,196 who have shown that primary-growing-chain-end epimerization occurs with exchange of the methylene and methyl carbons of the stereoinverted unit (A f B in Scheme 33). Further investigation is obviously required to assess if, and which one of these two mechanisms produces the epimerization of a growing chain end.

VI. Statistics of Polymerization A. General Remarks The polymerization reaction is a sequence of different events, such as monomer insertions, site isomerizations, and chain release reactions. The polymer chain can be seen as a permanent picture of the sequence of these events, and it is possible to use a statistical approach to study their distribution along the chain to increase our knowledge on polymerization mechanisms. As a consequence, a mathematical model of the polymerization can be built by assigning a probability at each event in our system. In the case of propene homopolymerization, this approach is (largely) used to study the mechanisms governing the stereoselectivity of the catalyst from the 13C NMR spectrum of the polymer. In fact, the type and the relative amount of the stereosequences present in the chain are obtained from the methyl region of the spectrum and are usually determined at the pentad level (see section II.G). This distribution can be studied using insertion probabilities for propene enantiofaces, which depend on the type of stereocontrol mechanism active for the catalytic

system. Due to the large variety of achievable structures, metallocene-based catalysts have been used to develop and to test these statistical models. A rationalization was made by Farina459 that divided metallocenes in different classes according to their symmetry and indicated possible statistical models to describe the polymerization behavior of each class. In the following sections this classification is also taken into account. Metallocene are characterized by the presence of two catalytic sites (see section II) for monomer insertion (Scheme 10 and related discussion), and the polymer chain migrates from one site to the other at each monomer insertion. The statistical parameters for enantioface selection for the two sites could be equal or different, depending on the symmetry elements present in the complex.459 In some cases, a “site isomerization” (i.e. a chain migration from a site to the other without monomer insertion) can occur. When this mechanism influences the stereosequence distribution (as is the case in syndiotactic polymerizations), the statistical model should contain a probability parameter to describe this event.

B. Mechanisms of Stereocontrol and Statistical Models As reported in section II.E, the two main mechanisms of stereocontrol in 1-olefin polymerization arise from the chirality of the catalytic site (enantiomorphic site control) and from the chirality of the last methine in the polymer chain (chain-end control). Two statistical models, based on these basic mechanisms, have been developed and used by different authors and are known as the enantiomorphic site model296 and the Bernoullian model.460


Chemical Reviews, 2000, Vol. 100, No. 4 1313 Table 12. Pentad Fractions for the Symmetric First-Order Markov Model

Scheme 34

pentad

probability expressions

mmmm mmmr rmmr mmrr rmrr

[1 - p(r)]4 2[p(r)][1 - p(r)]3 [p(r)]2[1 - p(r)]2 2[p(r)]2[1 - p(r)]2 2[p(r)]3[1 - p(r)]

pentad


rmrm rrrr rrrm mrrm

2[p(r)]2[1 - p(r)]2 [p(r)]4 2[p(r)]3[1 - p(r)] [p(r)]2[1 - p(r)]2

Scheme 35

monomer after a re inserted monomer is indicated as p(si|re). The other probabilities will be p(re|si), and p(si|si), p(re|re). Neglecting the presence of chain termination (high molecular weight polymers), we have the following relations between the probabilities:

p(re|si) + p(si|si) ) 1

1. Chain-End Control (Bernoullian Model) Tulleken,461

As indicated by there is a “nomenclature” problem with the term Bernoullian to indicate the chain-end model. As it is the last inserted monomer unit that controls the insertion of the following olefin, it would be better to use the term “symmetric” Markovian model. For achiral metallocene-based catalysts (C2v and achiral Cs metallocenes in Chart 2) the chain-end control is present as the only stereocontrol mechanism. It derives from the presence of an asymmetric carbon atom on the last inserted monomer. The chirality (R or S) of this atom is related to the enantiotopic face of the olefin where the insertion took place (Scheme 34). In the 13C NMR spectrum of the polymer we lose this kind of information, as two successive insertions of the re olefin face and two successive insertions of the si face produce the same m diad (see section II.G). As a consequence, we can observe only the relative chirality between consecutive inserted monomer units (S,S or R,R as m diads and S,R or R,S as r diads) disregarding the absolute configuration of tertiary atoms. We prefer to use the re and si nomenclature indicating the stereochemistry of the methines in the polymer chain (Scheme 35), bearing in mind that the insertion of the re propene enantioface will produce an S configuration on the methine. The approaching monomer experiences the chirality of the last inserted monomer and therefore four options are possible:

1 2 3 4

last inserted monomer

approaching monomer

resulting diad

re si re si

si re re si

r r m m

As the catalyst does not possess any chirality (and neglecting any penultimate effect), the two first alternatives are “mirror images” (see Scheme 35) and they must be equiprobable. The same is true for re-re and si-si options. The statistical modeling of experimental pentad distribution can be performed by considering a first-order Markovian model. The insertion probability for a si

p(si|re) + p(re|re) ) 1

The aforementioned equiprobabilities between some of the possible insertions can be expressed using the symmetric first-order Markovian model where the following relations are applied:

p(re|si) ) p(si|re) ) p(r) p(si|si) ) p(re|re) ) p(m) where p(r) is the probabilty of formation of r diads and p(m) is the probability of formation of m diads. Only one probability is independent (p(r), for example) while the other is given by

p(m) ) 1 - p(r) As no effect of the penultimate inserted unit is taken into account, the formation of r or m diads is a random process that follows the Bernoullian statistic. Therefore, the symmetric first-order Markovian model becomes Bernoullian when diad formation is considered. This fact explains why this model is usually called Bernoullian. The resulting probability expressions for pentad distribution are collected in Table 12. This model was employed by Ewen22 to describe the low-temperature polymerization with Cp2TiPh2/MAO catalyst.

2. Enantiomorphic Site Control Catalysts based on metallocenes belonging to the C2, prochiral Cs, and C1 classes (see Chart 2) are, in principle, able to direct 1-olefin insertion. Isotactic Control. Olefin insertion in C2-symmetric metallocenes occurs preferentially with the same face at both sites leading to an isotactic polymer. The isotacticity of the polymer chain depends on the metallocene structure. The chain-end control can be active, but for highly isotactic polymers it is difficult to check its presence as the pentads representing two consecutive wrong insertions have too low an intensity for a correct evaluation. When the chain-end control can be neglected, we can define the probability of insertion of the preferred olefin face using one probability parameter, b. As in the 13C NMR spectrum, we can observe only the stereochemical relation between contiguous units in terms of m and r diads, we can arbitrarily express b


Resconi et al.

Table 13. Pentad Fractions for an Isotactic Bernoullian Model pentad


pentad


mmmm mmmr rmmr mmrr rmrr

b5 + (1 - b)5 2[b4(1 - b) + b(1 - b)4] b3(1 - b)2 + b2(1 - b)3 2[b4(1 - b) + b(1 - b)4] 2[b3(1 - b)2 + b2(1 - b)3]

rmrm rrrr rrrm mrrm

2[b3(1 - b)2 + b2(1 - b)3] b3(1 - b)2 + b2(1 - b)3 2[b3(1 - b)2 + b2(1 - b)3] b4(1 - b) + b(1 - b)4

as the probability of an olefin insertion with the re enantioface (the insertion probability of a si olefin face will be 1 - b). The resulting isotactic Bernoullian model (this is a true Bernoullian model, as the chain end effect is neglected) gives the calculated pentad distribution reported in Table 13. Each pentad contains two symmetric contributions; for example, the mmmm pentad derives from the two possibilities

re-re-re-re-re all insertions are correct b5 mmmm ) + si-si-si-si-si all insertions are wrong (1 - b)5 The pentad fraction distribution is symmetric with respect to b ) 0.5, as we obtain the same values using b ) p or b ) 1 - p (0 e p e 1); therefore, we could add the restriction 0.5 e b e 1. It would be easy to show that this model satisfies the ratio [mmmr]:[mmrr]:[mrrm] ) 2:2:1 as experimentally found for isotactic polymers. This model can be applied to evaluate the pentad distribution from 13C NMR spectra of metallocenebased isotactic polypropenes in which overlapping with peaks from end group or regioirregular units (2,1 and 3,1) occurs.232 In this case only the peaks of mmmm, mmmr, mmrr, and mrrm pentads can be obtained from direct spectrum integration. Furthermore, the mmmr peak overlaps with the mmmm base, and the mmrr has to be correct by subtracting the contribution from the 2,1-erythro unit. The total pentad distribution is calculated under the hypothesis that the polymerization statistic follows a pure enantiomorphic site control, using the expressions reported in Table 13 as follows:

mmmm + mmmr ) b5 + (1 - b)5+2[b4(1 - b) + b(1 - b)4] mmrr ) 2[b4(1 - b) + b(1 - b)4] mrrm ) b4(1 - b) + b(1 - b)4 The best fit between experimental and calculated areas is searched through a least-squares method, minimizing the function: 2 f(b) ) ∑(Aexp - kAcalc i i ) i

where the sum is extended over the three groups of are the experimental areas, Acalc are pentads, Aexp i i

Scheme 36

the calculated ones, and k is a normalization constant calculated at each minimization step as:

k(b) )

exp exp + Aexp Ammmm+mmmr mmrr + mrrm calc calc Ammmm+mmmr + Acalc mmrr + Amrrm

Enantiomorphic Site with Chain-End Control. In the case of less stereoselective C2-symmetric metallocene catalysts, the magnitude of chain-end control can be comparable to that of site control. In this case, obviously, the former has to be added to the model using Markovian statistics. The probability parameters are the same found for pure chain-end control: p(si|re), i.e., the probability of insertion of a si monomer enantioface after a monomer inserted with the re face, p(re|si), p(si|si), and p(re|re). In this case, the metallocene chirality prevents the equiprobability of the si olefin insertion after a re inserted monomer (see structure on the left in Scheme 36) and re olefin insertion after a si inserted monomer (see structure on the right in Scheme 36). Furthermore, one of the two enantiofaces of the monomer is preferred for insertion (right picture in Scheme 36) with respect to the other. Only two probability parameters are independent as the other two are linked by the following relations:

p(si|re) + p(re|re) ) 1

p(re|si) + p(si|si) ) 1

The parameter p(re|re) represents the isospecific propagation at this site (two successive re insertions), while p(re|si) is the probability of “error correction” after a stereoerror. This model is called the asymmetric Markovian model and the mathematical expressions for pentad distribution are collected in Table 14. Syndiotactic Control. In prochiral Cs-symmetric metallocenes (Chart 2), one site favors the insertion of an olefin enantioface and the other preferably inserts the opposite one and a syndiotactic polymer is obtained. If we neglect the effect of the chain end, we can use a statistical model analogous to that shown for isotactic polymers with a different definition of the probability parameter. In this case the probability of insertion of a monomer with a given enantioface at site 1 is equal to the probability of insertion of a monomer with the opposite enantioface at site 2. This probability is indicated with the parameter a. The resulting expressions for pentad distribution for the syndiotactic Bernoullian model are reported in Table 15.



Table 14. Pentad Fractions with the Asymmetric Markovian Model pentad


mmmm mmmr rmmr mmrr mmrm + rmrr rmrm rrrr rrrm mrrm

{p(re|si)p(re|re)4

+ [1 - p(re|re)][1 - p(re|si)]4}/[1 - p(re|re) + p(re|si)] 2{p(re|re)3p(re|si)[1 - p(re|re)] + [1 - p(re|si)]3[1 - p(re|re)]p(re|si)}/[1 - p(re|re) + p(re|si)] {p(re|si)p(re|re)2[1 - p(re|re)]2 + [1 - p(re|si)]2[1 - p(re|re)]p(re|si)2}/[1 - p(re|re) + p(re|si)] 2{p(re|si)2p(re|re)2[1 - p(re|re)] + [1 - p(re|si)]2[1 - p(re|re)]2p(re|si)}/[1 - p(re|re) + p(re|si)] 2{[1 - p(re|si)]p(re|re)2[1 - p(re|re)]p(re|si) + [1 - p(re|si)]2p(re|re)[1 - p(re|re)]p(re|si) + p(re|re)p(re|si)2[1 - p(re|re)]2 + [1 - p(re|si)][1 - p(re|re)]2p(re|si)2}/[1 - p(re|re) + p(re|si)] 2{p(re|re)[1 - p(re|si)][1 - p(re|re)]p(re|si)2 + p(re|re)[1 - p(re|si)][1 - p(re|re)]2p(re|si)}/[1 - p(re|re) + p(re|si)] {p(re|si)3[1 - p(re|re)]2 + p(re|si)2[1 - p(re|re)]3}/[1 - p(re|re) + p(re|si)] 2{p(re|si)2p(re|re)[1 - p(re|re)]2 + [1 - p(re|si)] [1 - p(re|re)]2p(re|si)2}/[1 - p(re|re) + p(re|si)] {p(re|si)2p(re|re)2[1 - p(re|re)] + [1 - p(re|si)]2[1 - p(re|re)]2p(re|si)}/ [1 - p(re|re) + p(re|si)]

Table 15. Calculated Pentad Distribution with the Syndiotactic Bernoullian Model pentad


pentad


mmmm mmmr rmmr mmrr mmrm

a3(1 - a)2 + a2(1 - a)3 2a2(1 - a)3 + 2a3(1 - a)2 a4(1 - a) + a(1 - a)4 2a4(1 - a) + 2a(1 - a)4 2a3(1 - a)2 + 2a2(1 - a)3

rmrr rmrm rrrr rrrm mrrm

2a3(1 - a)2 + 2a2(1 - a)3 2a3(1 - a)2 + 2a2(1 - a)3 a5 + (1 - a)5 2a(1 - a)4 + 2a4(1 - a) a2(1 - a)3 + a3(1 - a)2

The pentad fraction distribution is symmetric with respect to a ) 0.5, as we obtain the same values using a ) p or a ) 1 - p (0 e p e 1). It would be easy to show that this model satisfies the ratio [rmmr]: [mmrr]:[rrrm] ) 1:2:2, as experimentally observed for syndiotactic polymers obtained with “sitecontrolled” catalysts. The presence of isolated m diads in 13C NMR spectra (rrmr pentad) of these polymers is attributable to an isomerization of the site due to a “skipped” monomer insertion during chain growth (see section IV.B). This fact can be included in the model considering the probability of site isomerization pbs, in the expressions for pentad distribution. The final result is shown in Table 16. Elastomeric Polypropene. To model the propene polymerization to elastomeric polypropene catalyzed by bis(2-arylindenyl)zirconocenes370 or C1-symmetric Me2X(Cp)(1-Ind) metallocenes,393 the consecutive twostate model originally described by Coleman and Fox376,377 was used. Due to the complexity of the model and the limited number of experimental points (only the nine pentad areas) used in the fitting procedure, it was not possible to unambiguously determine the overall propagation mechanism that leads to the microstructure of these polymers. Information on the relative abundance of isotactic and atactic blocks can be obtained using a statistical modeling of pentad distributions based on a competitive two-site model378,379 based on a mixing of a chainend-controlled site (to model the atactic blocks) and an enantiomorphic site (for isotactic blocks).365

if we need to fit longer stereosequences. To overcome these problems, an interesting method was reported by Busico and Vacatello.462 This is based on the Markov chain matrix mathematics, and we report it here as an interesting and powerful system to easily solve complicated problems. First, we have to found the “stochastic” matrix, that is, the matrix containing all the probability parameters of our model. The matrix for the syndiospecific polymerization with site isomerization is chosen as an example: A) re(1) si(1) re(2) si(2)

|

re(1) apbs apbs a(1 - pbs) a(1 - pbs)

si(1) (1 - a)pbs (1 - a)pbs (1 - a)(1 - pbs) (1 - a)(1 - pbs)

re(2) (1 - a)(1 - pbs) (1 - a)(1 - pbs) (1 - a)pbs (1 - a)pbs

si(2) a(1 - pbs) a(1 - pbs) apbs apbs

|

The rows are indexed to the last-inserted monomer enantioface (re or si) and to the site where insertion took place (1 or 2). The columns are indexed to the enantioface of the inserting unit and to the site. The probability parameters have the same meaning as in the aforementioned syndiotactic model. When the preferred enantioface for site 1 is re, then for site 2 it is si, and vice versa. If pbs ) 0, no site isomerization is present and the model collapses to the normal syndiospecific one. The sum of the elements in a row must be equal to one, i.e., monomer insertion has to occur! To evaluate the sequence (pentad) distribution we obtaine the two matrixes Am and Ar defined as Am ) re(1) si(1) re(2) si(2) Ar )

|

re(1) apbs 0 a(1 - pbs) 0

si(1) 0 (1 - a)pbs 0 (1 - a)(1 - pbs)

re(2) (1 - a)(1 - pbs) 0 (1 - a)pbs 0

si(2) 0 a(1 - pbs) 0 apbs

|

|

re(1) 0 apbs 0 a(1 - pbs)

si(1) (1 - a)pbs 0 (1 - a)(1 - pbs) 0

re(2) 0 (1 - a)(1 - pbs) 0 (1 - a)pbs

si(2) a(1 - pbs) 0 apbs 0

|

C. Use of Matrix Multiplication Methods in Statistical Models

re(1) si(1) re(2) si(2)

All the models described above imply the obtainment of a series of equations to express the pentad fractions as a function of the probability parameters. This could be a problem if models with a relatively high number of independent parameters are used or

They represent the probability of having, respectively, m or r diads along the polymer chain. The probability of a given stereosequence d1d2d3...dn, (di ) m for meso diads and di ) r for racemic diads)


Resconi et al.

Table 16. Pentad Fractions for a Syndiospecific Bernoullian Model in Presence of Site Isomerizationa pentad mmmm mmmr rmmr mmrr xmrxc rmrm rrrr rrrm mrrm

E0 p3 2p3 p4 2p4 4p3 2p3 p5 2p4 p3

E1

E2

E3

4p3 2(2p4 + 2p3) 4p3 2(p4 + 3p3) 2(p5 + 3p4 + 4p3) 2(p4 + 3p3) 2(p4 + p3) 2(p5 + p4 + 2p3) 2(p4 + p3)

3p4 + 3p3 2(p4 + 5p3) p5 + 2p4 + 3p3 2(p5 + p4 + 4p3) 2(4p4 + 8p3) 2(p5 + 3p4 + 2p3) 3p4 + 3p3 2(p4 + 5p3) p5 + 2p4 + 3p3

2p4 + 2p3 2(p5 + p4 + 2p3) 2(p4 + p3) 2(p4 + 3p3) 2(p5 + 3p4 + 4p3) 2(p4 + 3p3) 4p3 2(2p4 + 2p3) 4p3

E4

totalb

p5 2p4 p3 2p4 4p3 2p3 p3 2p3 p4

E0(1 - pbs) + E1(1 - pbs)3pbs + E2(1 - pbs)2pbs2 + E3(1 - pbs)pbs3 + E4pbs4 4

a p5 ) a5 + (1 - a),5 p4 ) a4(1 - a) + a(1 - a),4 and p3 ) a3(1 - a)2 + a2(1 - a)3. b pbs is the probability of site isomerization (see text); only the first expression is reported as the others are identical. c xmrx ) mmrm + rmrr.

is given by the matricial multiplication

f(d1d2d3...dn) ) f0TA1A2A3....AnJ where the matrix Ai ) Am when di ) m and Ai ) Ar when di ) r, J ) |1111|T, and f0T is a row vector and represents the vector of “stationary probabilities” of the four states and is evaluated by numerically solving the system of equations

f0TA ) f0T Or from the relation

lim An ) F nf∞ where each row of the F matrix is the vector f0T (in some models, the elements of f0T can be obtained also as a function of the probability parameters). The probabilty of the rrmmrrr heptad can be obtained easily as

f(rrmmrrr) ) 2f0TArArAmAmArArArJ

VII. Regiocontrol One of the features of most isospecific metallocene catalysts is their generally lower regioselectivity compared to heterogeneous Ziegler-Natta catalysts: indeed, despite the fact that primary propene insertion is clearly favored by electronic factors (see section III.E), isolated secondary propene units are often detectable in i-PP samples and their presence is the signature of a metallocene catalyst. These regiodefects have a strong effect in the lowering crystallinity and melting point of i-PP. At the same time, there is also a close correlation between catalyst regioselectivity on one side and catalyst activity and polymer molecular weight on the other, due to the lower monomer insertion rate at a secondary growing chain end and the competing β-H transfer to the monomer after a secondary insertion. Because of these two aspects, understanding the factors controlling the regioselectivity of a metallocene catalyst is important for catalyst design. The characterization of the different regiodefects by 13C NMR and the molecular modeling studies performed on the subject are discussed in section VII.A. The relative amounts of these regiodefects are highly dependent on the metallocene ligand structure and the polymerization conditions employed (polymerization temperature

and monomer concentration). Unfortunately, possibly due to the often low concentration of regioerrors and the requirement of a high-field NMR instrument and long acquisition times, few detailed studies have been carried out on the regioselectivity of metallocene catalysts and the dependence of the type and amounts of regioerrors on ligand structure and polymerization conditions. The available data are discussed in sections VII.C-E. The lower reactivity of a secondary growing chain with respect to a primary growing chain has been confirmed in three ways: by studing the activating effect of hydrogen (see section IX), by copolymerization with ethene,126-129 and by end group analysis.130,131 The latter two aspects are discussed in section VII.B. In section VII.F we discuss the proposed mechanisms of isomerization of a secondary growing chain into a 3,1 unit.

A. Stereochemistry of Regioirregular Insertion 1. 13C NMR Analysis As briefly seen in section II.F, secondary propene insertions, currently referred to as 2,1 insertions, occur in i-PP from isospecific metallocene catalysts with high but opposite (with respect to primary insertions) enantioface selectivity (Scheme 14). 13 C NMR analysis has shown that these 2,1 units are always isolated between two isotactic blocks and, depending on the enantioselectivity of the following primary insertion on the secondary growing-chain end, give rise to 2,1 erythro (e) and 2,1 threo (t) sequences and to the formation of tetramethylene sequences (3,1 units), arising from the unimolecular isomerization of the secondary unit (Scheme 37). The stereochemical environment of the secondary 2,1 units and of the 3,1 unit have been assigned by 13C NMR analysis of i-PP made with rac-C2H4(1-Ind)2ZrCl2 and rac-C2H4(H4-1-Ind)2ZrCl2.117,126,198,199,463 A previous misassignment of the structure of the 2,1 threo unit117,198 has been corrected by Mizuno.199,304 A sequence of two secondary insertions has never been detected. Thus, in terms of regiochemistry, three propagation reactions occur, i.e., primary on primary chain end, secondary on primary chain end, and primary on secondary chain end. Because of the absence of sequences of two secondary insertions, the last two values must be equal, that, is the probability of primary insertions on primary chain ends (pp) obeys the relationship pp ) 1-2ps, where ps is the number of primary insertions on secondary chain ends, estimated from the intensity of the 13C NMR



Scheme 37

Chart 20. Chain Microstructure Defects Generated by Isolated Secondary (2,1) Insertion: erythro (meso), threo (racemic) Secondary Units and 3,1 Unita

Figure 39. 13C NMR spectra (100 MHz, C2D2Cl4, 120 °C, ref mmmm at 21.8 ppm) of three i-PP containing regioirregularities: from rac-C2H4(H4-1-Ind)2ZrCl2/MAO (top); from rac-C2H4(4,7-Me2-1-Ind)2ZrCl2 (middle); from rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2 (bottom). For carbon numbering, see Chart 20 and Table 2. For polymerization conditions, see Table 17. cis-Bu ) cis-2-butenyl, n-Pr ) n-propyl.

primary insertion following the regioerror) are far more frequent (in liquid propene polymerization) than primary stereoerrors. The regioselectivity of propene insertion into a Zr-H bond (initiation) is discussed in section IX.

2. Molecular Modeling

a The saturated end group due to chain start lays on the right of the chain segment, the unsaturated one generated by chain release on the left.

methylene peaks t9 and e9 (see carbon labeling in Chart 20). The 13C NMR spectra of three i-PP containing regioirregularities of the three types are shown in Figure 39,while the relevant chain segments are reported in Chart 20. rac-C2H4(H4-1-Ind)2ZrCl2/MAO (Figure 39 (top)) produces i-PP with almost only 3,1 units (∼1%), while rac-C2H4(4,7-Me2-1-Ind)2ZrCl2 (Figure 39 (middle)) produces a lower molecular weight i-PP with a higher amount (∼2%) of both e and t 2,1 units. The Targor catalyst rac-Me2Si(2-Me4-Ph-1-Ind)2ZrCl2 (Figure 39 (bottom)) represents a limit situation in which secondary insertions (∼1%, only e type, indicating a highly enantioselective

Before discussing the correlation between the regioselectivity of a given metallocene and its symmetry, we start to discuss the enantioselectivity of regioirregular insertions in isospecific polymerization. As seen above, 13C NMR analysis of the stereochemical environment of a secondary unit shows that the regioirregular (secondary) propene insertion is highly enantioselective. The modeling tools used to investigate the enantioselectivity of primary insertions can be used also to investigate the enantioselectivity of secondary insertions. Nonetheless, this kind of analysis was somewhat overlooked in the first studies on the enantioselectivity of primary propene insertions. Corradini,263 Rappe´,147 and Morokuma155 and their co-workers could have sentenced (but they did not) that for a C2-symmetric catalyst secondary insertions are enantioselective. However, the authors focused most on the enantioselectivity of primary insertions263 or on the steric effects favoring primary over secondary insertion147,155 and did not discuss the enantioselectivity of secondary insertion. The first detailed analysis on the enantioselectivity of secondary propene insertion on isospecific C2symmetric catalysts was performed by Corradini and co-workers.200 The models reported in Figure 40 represent energy minima corresponding to coordination of propene suitable for secondary insertion into the model complex based on the (R,R)-Me2Si(1-Ind)2 ligand. Structure A, with a re-coordinated propene, is higher by nearly 5 kcal/mol with respect to structure B, with a si-coordinated propene. The


Resconi et al.

Figure 40. Models for secondary propene insertion into a primary polypropene growing chain, when the aromatic ligand is Me2Si(1-Ind)2. A and B correspond to re and si propene coordinations, respectively. Model B, with a si coordinated propene, is the only one suitable for monomer insertion.200

Figure 41. Models for the secondary propene insertion into a primary polypropene growing chain, when the aromatic ligand is Me2C(Cp)(9-Flu). A and B correspond to re and si propene coordinations, respectively. Model A, with a re coordinated propene, is the only one suitable for monomer insertion.91

higher energy of the model with the re-coordinated propene is due to the repulsive interactions of the methyl group of propene with the six-membered rings of one of the indenyl ligands. Similar conclusions have been obtained with model complexes containing the (R,R)-C2H4(H4-1-Ind)2 and (R,R)-Me2C(1-Ind)2 ligand.91,200 It is worth noting that the enantioselectivity in the secondary insertion is due to direct interaction of the monomer with the ligand, while the growing chain plays no role, whereas the enantioselectivity in the primary insertion is due to the chiral orientation of the growing chain (forced by the ligand) and that direct interaction of the monomer with the ligand is, in general, negligible. Moreover, for a (R,R) coordination of the aromatic ligand, the re propene enantioface is favored for primary insertion, on models of the type described above. In short, the C2-symmetric isospecific models above-mentioned are substantially enantioselective for the lower energy (and experimentally observed) primary monomer insertion as well as for the higher energy (experimentally detected) secondary monomer insertion. Anyhow, it is worth noting that the enantioselectivity of the isospecific model site is in favor of opposite monomer prochiral faces, for primary and secondary insertions.200 This result is in perfect agreement with the observed microstructure of polypropene chains obtained by isospecific catalytic systems including the aforementioned ligands, as discussed in section VII.1, and it is consistent with the in nuce results of Corradini,263 Rappe´,147 and Morokuma155 mentioned above. Furthermore, for models of catalyst based on the (R,R)-C2H4(1-Ind)2 and (R,R)-C2H4(H4-1-Ind)2 ligands, a detailed molecular mechanics analysis has been conducted also for the case of primary or secondary propene insertions on secondary polypropene chains (for which the last propene insertion has been secondary) by Corradini, Guerra, and co-workers200 and by Yu and Chien.276 According to this analysis, the enantioselectivities observed after an occasional secondary monomer insertion are easily accounted for in the framework of the mechanism of the “chiral orientation of the growing chain”. In fact, substituting the usual primary growing chain with a secondary

growing chain reduces the preference for one particular chiral orientation of the growing chain, since the first C atom of a secondary chain bears two C atoms (the Cβ of the chain and the CH3 of the secondary inserted monomer) that can repulsively interact with the ligand’s framework. Moreover, a secondary growing chain corresponds to a reduced bulkiness of the substituents of the second carbon atom of the chain (which is secondary carbon for the secondary chain, but is tertiary carbon for the primary chain). Wherefore, the energy differences between models with the different propene enantiofaces is reduced, leading to a less pronounced enantioselectivity.200,276 These results are able to rationalize the probability distributions of stereochemical configurations of regioirregular units in isotactic polymer samples prepared in the presence of the corresponding catalytic systems.198,199 Finally, Yu and Chien also found that propene insertion on the secondary chain is of considerably higher energy, roughly 10 kcal/mol, relative to propene insertion on a primary chain, supporting the broadly accepted idea that after secondary propene insertion the polymerization is essentially stalled. The same modeling used to investigate the enantioselectivity of secondary insertion with C2-symmetric metallocenes was applied to the enantioselectivity of secondary insertion with Cs-symmetric metallocenes.91,277 Yu and Chien found that the same propene enantioface is favored for both primary and secondary propene insertions, on models based on the Me2C(Cp)(9-Flu) ligand, by roughly 2-3 kcal/mol.277 Corradini, Guerra and co-workers rationalized these findings using the models reported in Figure 41, which represent energy minima corresponding to coordination of propene suitable for secondary insertion into the model complex based on the Me2C(Cp)(9-Flu) ligand, with R chirality at the metal atom.91 As for the models of C2-symmetric metallocenes, the higher energy, about 5 kcal/mol,91 of the model with the unfavored secondary propene coordination (si in this case) is due to the repulsive interactions of the methyl group of propene with the bulkier moiety (the fluorenyl group in this case) of the ligand. In summary, also models for syndiospecific Cs-symmetric metallocenes are substantially enantioselective for the lower energy primary monomer insertion as well



Scheme 38145

as for the higher energy secondary monomer insertion. Anyhow, it is worth noting that differently from the isospecific C2-symmetric models, the enantioselectivity of the syndiospecific model site is in favor of the same monomer prochiral face, for primary and secondary insertions.91,277 Syndiospecific catalytic systems based on Cs-symmetric metallocenes are more regioselective than the C2-symmetric metallocenes of class II. As a consequence, the enantioselectivity in regioirregular insertions have been experimentally investigated for propene-based copolymers only.464,465 In particular, 13C NMR characterization of ethene-1-13C/propene copolymers suggests that the very low amount (0.030.07%) of regioirregular 2,1 units are substantially stereoirregular.464 On the contrary, NMR characterization of propene/styrene/ethene terpolymers has shown that insertions of propene (primary) and of styrene (secondary) occur with the same enantioface.465 The main conclusion of the previously reported experimental and theoretical studies is that opposite enantiofaces are favored for primary and secondary propene insertion on C2-symmetric metallocenes, whereas the same enantioface is favored for primary and secondary insertion on Cs-symmetric metallocenes. A further proof of this mechanistic picture has been obtained by Corradini and co-workers through a combination of primary and secondary propene insertions into one single insertion step, by using 2-butene as monomer.145 They argued that, within the above framework, insertion of Z-butene should be favored with C2-symmetric metallocenes, whereas insertion of E-butene should be favored with Cs-symmetric metallocenes, as sketched in Scheme 38, provided that the same steric interactions which rule the enantioselectivity of primary and secondary propene insertions hold for 2-butene. The QM/MM transition states for Z- and E-butene insertion into the Zr-C(n-propyl) σ-bond of the C2and Cs-symmetric Me2Si(1-Ind)2 and Me2Si(Cp)(9Flu) metallocenes are reported in Figure 42, parts A-B, and C-D, respectively. According to the mechanism of the chiral orientation of the growing chain, the n-propyl group used to simulate a polyethylenic growing chain assumes a conformation which mini-

Figure 42. QM/MM transitions states of 2-butene insertion reaction into the Zr-C σ-bond with the C2- and Cssymmetric metallocenes. Parts A and B correspond to Zand E-butene insertion with the C2-symmetric Me2Si(1Ind)2Zr(n-propyl)+ metallocene, respectively. Parts C and D correspond to Z- and E-butene insertion with the Cs-symmetric Me2Si(Cp)(9-Flu)Zr(n-propyl)+ metallocene, respectively. For clarity, the 2-butene C atoms are shaded.145

mize repulsive interactions with the metallocene ligand, and the head-methyl group is pushed to the opposite side relative to the growing chain to minimize steric interactions between the methyl group itself and the growing chain. This orientation of the head-methyl group implies that, for C2-symmetric metallocene, Z-butene insertion is favored relative to E-butene insertion, since for Z-butene the tail-methyl group is located far from the six-membered aromatic rings of the metallocene ligand. On the contrary, for Cs-symmetric metallocene, E-butene insertion is favored relative to Z-butene insertion, since for Ebutene the tail-methyl group is located far from the six-membered aromatic rings of the metallocene ligand.145 These predictions were confirmed by ethene/2butene copolymerizations. In fact, C2-symmetric metallocenes scarcely insert, less than 2%, E-butene, while they insert relevant fractions, 25%, of Z-butene; analogously, Cs-symmetric metallocenes insert less than 2% of Z-butene, while inserting 14% of Ebutene.145

B. Reactivity of a Secondary Growing Chain 1. Copolymerization with Ethene When low amounts of ethene are added to a propene polymerization with a non-fully-regioselec-


Resconi et al.

Scheme 39

Scheme 40

tive catalyst, a series of mechanistic information can be obtained. The use of 13C-labeled ethene allows the detection of very low amounts of regioerrors.126,464,466 In addition, to provide further proof for site versus chain-end control, 13C NMR analysis of isotactic propene/ethene copolymers made with rac-C2H4(1Ind)2ZrCl2/MAO and rac-C2H4(H4-1-Ind)2ZrCl2/MAO has given the following results (Scheme 39): (i) Ethene inserts preferentially after a secondary propene insertion. This means that primary propene insertion on a secondary growing chain is slower than on a primary growing chain: skp < pkp. Indeed, small amounts of ethene added to a rac-C2H4(1-Ind)2ZrCl2/ MAO-catalyzed propene polymerization increase catalyst activity.304 For the same reason, ethene generates an increase in molecular weight, by reducing chain release after a secondary insertion.129 (ii) The enantioselectivity of the secondary insertion is confirmed to be high and opposite to that of primary insertion. (iii) The fact that the -(CH2)4- chain segment is produced by a 3,1 propene insertion is confirmed. (iv) The 2,1 f 3,1 isomerization is faster than ethene insertion, at least with the rac-C2H4(1-Ind)2ZrCl2/ MAO catalyst, arguing in favor of a unimolecular isomerization mechanism.126

2. Chain Release Chain release after a 2,1 insertion has been already described in section III.F. Recent studies on chain release mechanisms have shown that, when even low amounts of secondary insertions occur, 2-butenyl end groups become relevant, and often more frequent than the vinylidene end group. This has been observed, for example, in C2-symmetric ansa-zirconocenes such as rac-C2H4(1-Ind)2ZrCl2, rac-C2H4-

(4,7-Me2-1-Ind)2ZrCl2,130 rac-C2H4(H4-1-Ind)2ZrCl2, and rac-Me2Si(Benz[e]ind)2ZrCl2.467 Almost always the stereochemistry of the terminal double bond is cis, and formation of the cis-2-butenyl end group has been attributed to β-H transfer to the monomer after a secondary insertion. Unimolecular chain release would be expected to produce a trans-butenyl end group (Scheme 40).130 This end group has been detected in a PP sample produced in liquid monomer by the low activity rac-C2H4(4,7-Me2-H4-1-Ind)2ZrCl2 catalyst.131 The exceptions are ansa-zirconocenes with 2-methylindenyl ligands, for which chain release after a secondary unit is effectively suppressed.154,467

C. Regioselectivity: Influence of the Catalyst Structure 1. Influence of the Metal The influence of the metal can be gathered by comparing the results reported in ref 466 with those of refs 304 and 468: rac-C2H4(1-Ind)2TiCl2/MAO is both less stereoselective and less regioselective than its Zr analogue. rac-C2H4(1-Ind)2HfCl2/MAO gives i-PP which is very similar to that obtained with Zr. Other ligands seem to give the opposite effect: racMe2Si(2-Me-4-Ph-1-Ind)2TiCl2/MAO has been reported to be more regioselective than its Zr and Hf analogues.469 There are however too few examples of stereoselective Ti and Hf complexes to allow a good comparison, mainly due to the generally lower activity of the Hf and Ti complexes.

2. Influence of the Cocatalyst There is some controversy about the influence of the type of cocatalyst on either the stereo- or regio-



Table 17. Propene Polymerization with Zirconocene/MAO Catalysts: Synthesis and Microstructurea tacticityb zirconocene C2 (racemic) CH2(1-Ind)2ZrCl2 Me2C(1-Ind)2ZrCl2 Me2C(H4-1-Ind)2ZrCl2 C2H4(1-Ind)2ZrCl2e C2H4(H4-1-Ind)2ZrCl2 C2H4(4,7-Me2-1-Ind)2ZrCl2e C2H4(4,7-Me2-H4-1-Ind)2ZrCl2 CH2(2-Me-1-Ind)2ZrCl2 Me2Si(1-Ind)2ZrCl2 Me2Si(H4-1-Ind)2ZrCl2 Me2Si(2-Me-1-Ind)2ZrCl2 Me2Si(4-Ph-2-Me-1-Ind)2ZrCl2g C2H4(3-Me-1-Ind)2ZrCl2 Me2C(3-t-Bu-1-Ind)2ZrCl2 C1 (isospecific) Me2C(3-t-Bu-Cp)(9-Flu)ZrCl2h Me2C(3-t-Bu-Cp)(9-Flu)HfCl2

Al/Zr, molar ratio A, kg/(mmolZr h) 4 000 3 000 3 000 8 000 20 000 2 000 2 000 2 000 3 000 8 000 4 000 10 000 8 000 8 000 nr nr

62 66 37 140 37 72 5 56 17 54 33 1300 28 125 23 nr

bobsd

regioinversions (%)c

% mmmm 2,1 e 2,1 t

3,1

0.9349 0.9580 0.9916 0.9736 0.9824 0.9831 nd 0.9145 0.9798 0.9896 0.9882 0.9991 0.7233 0.9894

71.40 80.69 95.88 87.47 91.50 91.84 nd 63.96 90.30 94.91 94.25 99.55 19.96 94.80

0.26 0.20 0.31 0.35 0.11 1.29 0.0 0.24 0.27 0.14 0.33 0.46 0 0

0.21 0.0 0.18 0.0 0.09 0.18 0.19 0.01 0.06 0.80 0.45 0.05 0.0 18.9 0.11 0.0 0.21 0 0.00 0.39 0 0 0 0 0 0 0 0

-

77.47 72.39

nd nd

nd nd

0.44 0.55

end groups total

ref

0.47 0.38 0.58 0.55 0.97 2.80f 18.9 0.35 0.48 0.53 0.33 0.46 0 0

51 50 131 50 131 50 131 202 50 131 50 324 50 50

1

0 0

0.44 470 0.55 470

a Polymerization conditions: 1-L stainless steel autoclave, 0.4 L of propene, 50 °C, 1 h, zirconocene/MAO aged 10 min. Determined assuming the enantiomorphic site model, on primary insertions only; see ref 232. c Determined as described in ref 232; end groups not included. d In liquid monomer, bobs f b. e Average values. f End groups included. g Tp ) 70 °C. h Tp ) 60 °C.

b

selectivity of insertion.46,70,71-77 We observed that by changing the cocatalyst from MAO to isobutyl alumoxane in rac-C2H4(1-Ind)2ZrCl2 catalyzed polymerization of liquid propene, the resulting i-PP have virtually identical microstructures even in the type and amount of regioerrors, although catalyst activity varies considerably. Since the role of MAO is discussed in detail by Chen and Marks in this issue of Chemical Reviews, this aspect is not further analyzed here.

3. Influence of the π-Ligands: Experimental Data The microstructure of low molecular weight a-PP from Cp2ZrCl2 has been studied in detail: no internal 2,1 units were detected by 13C NMR,214 while less than 1% of end groups are cis-2-butenyl, the rest being vinylidene.202 This suggests that Cp2ZrCl2 is highly regioselective and chain propagation cannot proceed after an occasional secondary insertion. Similarly, the more active (MeCp)2ZrCl2 produces a-PP oligomers without detectable internal 2,1 units and 3.6% 2-butenyl end groups.131 The propene oligomerization catalysts (Me5Cp)2ZrCl2 and its Hf analogue are even more regioselective, since secondary units could not be detected even as chain ends. The Me2Si(9-Flu)2ZrCl2 catalyst for high molecular weight atactic polypropene is of similar high regioselectivity. (Ind)2ZrCl2 and (H4Ind)2ZrCl2/MAO catalysts, on the contrary, produce low molecular weight a-PP with about 1% of 3,1 units and about 10% 2-butenyl end groups. Both the Cs-symmetric syndiospecific zirconocenes Me2C(Cp)(9-Flu)ZrCl2 and Ph2C(Cp)(9-Flu)2ZrCl2 show, in liquid propene polymerization, no detectable 2,1 units. However, by using 1-[13C]-ethene in propene polymerization with a similar catalyst system, Busico was able to detect e0.08% 2,1 units.464 C2-symmetric zirconocenes show the greatest variability in terms of both stereo- and regioselectivities, with total regioerrors ranging from almost 20% in

PP (practically a 1,2-propene-co-3,1-propene alternating copolymer) made with rac-C2H4(4,7-Me2-H4-1Ind)2ZrCl2341 down to virtually zero in the newest generation of highly isospecific zirconocenes developed by Montell.50,230,324 Due to the presence of growing-chain-end isomerization reactions and to the fact that regioselectivity can be monomer concentration dependent (see section VII.D), as it is the case of isospecificity, also the regioselectivity of ansazirconocenes has to be evaluated at the same propene concentration and polymerization temperature and, if possible, in liquid monomer. Such a comparison is shown in Table 17 for a series of i-PP samples prepared with a wide range of bridged C2-symmetric zirconocene/MAO catalysts in liquid propene. Inspection of the data in Table 17 shows that hydrogenated ligands generate more stereoselective but less regioselective catalysts. The same effect occurs by methyl substitution on the frontal (4) position of indene: noteworthy, with respect to Brintzinger’s benchmark catalyst rac-C2H4(1-Ind)2ZrCl2/MAO, is the higher stereoselectivity but much lower regioselectivity of the rac-C2H4(4,7-Me2-1-Ind)2ZrCl2/MAO catalyst. Clearly the more open sites are less isospecific and more regioselective compared with sites bearing bulkier ligands. As seen in section V.A.1, C2-I-9, having a wider “bite angle” β (see Table 1), is slightly less isospecific but more regioselective than C2-I-1/ MAO: these findings are in agreement with, and explained by, the mechanistic model proposed by Guerra, which connects regioselectivity to stereoselectivity in propene polymerization (see section VII.C.5).91 Spaleck and co-workers have shown that combining hydrogenation and 4-methyl substitution, as in the rac-C2H4(4,7-Me2-H4-1-Ind)2ZrCl2/MAO catalyst with the 4- and 7-methyl groups endo oriented, results in a dramatic loss of regioselectivity.341 Another relevant effect of tetrahydroindenyl-based ansaligands is that the fraction of secondary units undergoing 2,1 f 3,1 isomerization is always much


higher compared to their indenyl-based analogues: in the case of rac-C2H4(4,7-Me2-H4-1-Ind)2ZrCl2, regioinversions are observed as 3,1 units only, indicating that, for this system, the site bearing a secondary growing chain end (secondary active site) cannot insert further monomer without previous isomerization to the linear chain (secondary growing-chainend isomerization). An interesting case is that of the highly active, highly stereoselective rac-Me2Si(2-Me4-Ph-1-Ind)2ZrCl2 catalyst, which produces, in liquid monomer, very high molecular weight i-PP with pentad content over 99% and about 0.5% 2,1 erythro units. In this case, secondary growing chains do not seem to influence catalyst activity, since in polymerizations carried out in the presence of ethene these 2,1 units are maintained in the polymer. The last, most recent class of C2-symmetric ansa-zirconocenes to be developed, of general formula rac-R2C(3-R-1Ind)2ZrX2, show a very high regioselectivity. For example, careful inspection of 100 MHz 13C NMR spectra of i-PP made with rac-Me2C(3-t-Bu-1-Ind)2ZrCl2 revealed no trace of 2,1 units (that is, given the signal-to-noise ratio in these spectra, a concentration lower than 0.02%) in the chain and none (2butenyl end groups < 1/200 000 units) in the chain end groups.50,230 Concerning C2-symmetric systems, the data shown in Table 17 are easily summarized by the following observations: (i) only the dimethyl-substituted bisindenyl derivatives (such as rac-C2H4(4,7-Me2-1-Ind)2ZrCl2 and rac-C2H4(4,7-Me2-H4-1-Ind)2ZrCl2) allow a significant degree of secondary insertions; (ii) hydrogenation of the indenyl moiety promotes a higher amount of 2,1 f 3,1 isomerization, (iii) substitution in C(2) increases regioselectivity as well as stereoselectivity; and (iv) substitution in C(3) produces highly regioselective catalysts, among the most regioselective of all metallocenes. Very few data are available concerning the regioselectivity of the aspecific, Cs-symmetric meso isomers of C2-symmetric racemic metallocenes: both meso-C2H4(1-Ind)2ZrCl2/MAO 91 and meso-Me2Si(2Ph-1-Ind)2ZrCl2/MAO were shown to produce a-PP free of regioerrors. 343 Also meso-Me2Si(2-Me-4-Ph1-Ind)2ZrCl2/MAO is much more regioselective (no regioerrors could be detected in the 100 MHz 13C NMR spectra of an a-PP sample prepared in liquid monomer at 70 °C) than rac-Me2Si(2-Me-4-Ph-1Ind)2ZrCl2/MAO.202 Waymouth’s catalyst, (2-Ph-Ind)2ZrCl2, is more regioselective than both (Ind)2ZrCl2 and rac-C2H4(1Ind)2ZrCl2; the same holds true for the analogue [2-(3,5-CF3)2C6H3)-Ind]2ZrCl2.372 For these systems, regioerrors are found only within the isotactic stereoblocks; thus, a secondary insertion can occur only when the catalyst is in an enantioselective conformation. C1-symmetric catalysts are slightly more regioselective compared to benchmark class II zirconocenes: Me2C(3-t-Bu-Cp)(9-Flu)ZrCl2 and Me2C(3-t-Bu-Cp)(9-Flu)HfCl2 show 0.44 and 0.55% 3,1 units respectively, at Tp ) 60 °C.470

Resconi et al.

Figure 43. Pseudotransition states for monomer orientations suitable for the favored primary (part A) and secondary (part B) propene insertions into a primary polypropene growing chain, for the case of the (R,R) coordinated C2symmetric rac-Me2Si(2-Me-1-Ind)2 ligand. Short nonbonded distances between the methyl group of propene and a methyl group of the ligand are indicated.272

4. Influence of the π-Ligands: Molecular Modeling Analysis Before relating about the conclusions obtained by different research groups, it has to be noted that the energy difference between the secondary and primary propene insertion, ∆Eregio, can be considered composed by two main contributions, electronic and steric. The electronic contribution has been treated previously, while here we focus on the steric contribution to ∆Eregio, due to steric interaction among the monomer, the growing chain, and the ligand skeleton. The substantial influence of methyl groups in position 2 of the ligand on the regioselectivity of the catalyst has been rationalized by Morokuma155,275 and Corradini272 and their co-workers. Their molecular mechanics calculations clearly show that the energy difference between secondary and primary propene insertion into the Zr-C(chain) σ-bond increases by roughly 1-5 kcal/mol with respect to the analogous energy difference of the corresponding unsubstituted parent ligand, when a methyl group is added on the C(2) position of the ligands, as in the H2Si(2,4-Me2Cp)2,155 H2Si(2-Me-1-Ind)2,275 Me2Si(2-Me-1-Ind)2,272 and Me2Si(2-Me-4-t-Bu-Cp)2272 ligands. The increased regioselectivity of 2-methyl-substituted catalysts is due to direct repulsive interaction of the methyl group of the monomer in a geometry suitable for secondary insertion, with the methyl group on C(2). The geometries of approximated transition states for primary and secondary propene insertion on the zirconocene based on the Me2Si(2Me-1-Ind)2 ligand are sketched in Figure 43. Clearly, model B, suitable for secondary insertion, presents one of the methyl groups in position 2 of the ligand at a short distance from the methyl group of the propene. On the contrary, the distances between the methyl substituents in position 2 of the ligand and the methyl group of propene are larger than 5 Å in model A, suitable for primary insertion. This accounts for the increased steric contribution to ∆Eregio calculated for catalysts based on ligands bearing methyl substituents in position 2.155,272,275 Moreover, Morokuma155,275 and Corradini272 and their co-workers also rationalized the reduced steric contribution to ∆Eregio when an alkyl group is present in position 4. Morokuma and co-workers calculations on the H2Si(4-i-Pr-1-Ind)2 ligand and on the H2Si(2Me-4-alkyl-1-Ind)2 ligands, with the alkyl group in



5. Relationship between Regioselectivity and Type of Stereoselectivity

Figure 44. Pseudotransition states for monomer orientations suitable for the favored primary (part A) and secondary (part B) propene insertions into a primary polypropene growing chain, for the case of the (R,R) coordinated C2symmetric rac-Me2Si(4,7-Me2-1-Ind)2 ligand. Short nonbonded distances between the methyl group of propene and a methyl group of the ligand are indicated.272

position 4 in the series methyl, ethyl, tert-butyl,275 predicted that the energy difference between secondary and primary insertions is reduced by 2-8 kcal/ mol, relative to the same energy difference with the parent 4-unsubstituted ligand. This holds also for the H2Si(2-Me-Benz[e]ind)2 ligand. In fact, the steric contribution to ∆Eregio for the catalyst based on this ligand has been calculated to be 3.5 kcal/mol lower than the steric contribution to ∆Eregio for the catalyst based on the H2Si(2-Me-1-Ind)2 ligand.275 The diminished regioselectivity of catalysts based on ligands containing methyl groups in position 4 (and 7) is due to direct repulsive interaction of the methyl group of the monomer in a geometry suitable for primary insertion, with the methyl group in position 4. The geometries of approximated transition states for primary and secondary propene insertion on the zirconocene based on the Me2Si(4,7-Me2-1Ind)2 ligand are sketched in Figure 44. Clearly, model A, suitable for primary insertion, presents one of the methyl groups in position 4 of the ligand at short distance from the methyl group of the propene. On the contrary, the distances between the methyl substituents in positions 4 and 4′ of the ligand and the methyl group of propene are larger than 5 Å in model B, suitable for secondary insertion. This accounts for the reduced steric contribution to ∆Eregio calculated for catalysts based on ligands bearing methyl substituents in position 4 (and 7).272 This feature is particularly enhanced when the methyl groups point toward the equatorial belt of the metallocene, as for the catalyst based on the (R,R) coordinated C2H4(4,7-Me2-H4-1-Ind)2 ligand with R chirality at the 4,4′ and S chirality at the 7,7′ C atoms. In this case, the steric contribution to ∆Eregio is lower by almost 2 kcal/mol relative to the steric contribution to ∆Eregio calculated for the catalyst based on the unsubstituted coordinated C2H4(H4-1Ind)2 ligand. As for the effect of an alkyl group substituted in position 3, Morokuma and co-workers275 predicted that a methyl substituent in position 3 of the Me2Si(1-Ind)2 ligand should increase the steric contribution to ∆Eregio, whereas Corradini and co-workers found that the same methyl group should slightly decrease this steric contribution.471 Finally, when the alkyl group substituted in position 3 is the bulky tertbutyl group, a higher steric contribution to ∆Eregio, roughly 2 kcal/mol, is predicted instead.272

Finally, we turn our attention to the relationship between regiospecificity and type of stereospecificity, that is, between the regioselectivity of a catalyst and its symmetry. In fact, as discussed above, syndiospecific and aspecific zirconocene catalytic systems are in general more regioselective than class II isospecific systems, most of which produce i-PP containing substantial amounts of regioirregular monomeric units, independently of the nature of the π-ligands and of the bridge between them (with the notable exception of 3-substituted bisindenyl systems). This dependence of the degree of regioselectivity on the symmetry rather than on the nature of the π-ligands and of the bridge between them (unless suitable substitutions of the ligands are involved) is not easy to rationalize by invoking differences in the electronic contributions to regioselectivity. On the other hand, the results discussed in the previous sections indicate that the steric contribution to the energy differences between secondary and primary propene insertion, for zirconocene-based catalytic models, is not greatly dependent on the symmetry of the π-ligands and hence on their stereoselectivity. However, the calculated energy difference between secondary and primary propene insertion for a given enantioface of the monomer is strongly dependent on the symmetry, and hence stereoselectivity, of the catalysts. In particular, for the syndiospecific Cs-symmetric catalysts a large steric contribution to regioselectivity is calculated for the enantioface which is wrong (subscript w in the following) for the primary insertion, whereas for the C2-symmetric isospecific catalysts a large steric contribution to regioselectivity is calculated for the enantioface which is right (subscript r in the following) for primary insertion. In short, the very same propene enantioface is favored for primary and secondary propene insertion with the Cs-symmetric syndiospecific catalysts,90,91,155,277 whereas opposite propene enantiofaces are favored for primary and secondary propene insertion with the C2-symmetric isospecific catalysts.91,147,155,200,276 On the basis of this modeling background, Corradini and co-workers developed a model able to relate regioselectivity on the type of stereoselectivity.91 For generic aspecific, syndiospecific, and isospecific model complexes, schematic plots of the internal energy versus the reaction coordinate, both for primary and secondary insertions, are sketched in Figure 45, parts A, B, and C, respectively. The minima at the center and at the ends of the energy curves correspond to propene-free intermediates including a growing chain with n and n + 1 monomeric units, respectively (just as an example, a possible geometry for these intermediate could correspond to the β-agostic geometry hypothesized as the resting state by several authors). Movements from the central minimum toward the left and the right correspond to possible reaction pathways leading to primary and secondary propene insertions, respectively. For the enantioselective complexes, the reaction pathways for monomer enantiofaces which are right and wrong for primary


Figure 45. Schematic plots of the internal energy versus the reaction coordinate, for both primary and secondary insertions, for generic aspecific (A), syndiospecific (B), and isospecific (C) model complexes. The minima at the centers and at the ends of the energy curves correspond to alkenefree catalytic intermediates including a growing chain with n and n + 1 monomeric units, respectively. Movements from the central minima toward the left and the right correspond to possible reaction pathways leading to primary and secondary insertions, respectively. For the enantioselective complexes (B,C), the reaction pathways for monomer enantiofaces being right (r) and wrong (w) for primary insertion are different and are indicated by full and dashed lines, respectively. The two energy barriers encountered for each pathway correspond to the coordination and insertion steps. The energy minima between the energy barriers for the monomer coordination and insertion correspond to alkene-bonded catalytic intermediates.91

insertion are different, and are indicated by full and dashed lines, respectively. For each pathway, the two energy barriers correspond to the coordination and insertion steps. The energy minima between the energy barriers for the monomer coordination and insertion, labeled as preinsertion intermediates in Figure 45, correspond to propene-bonded (π-complex) intermediates of the type used by several authors to discuss the enantioselectivity of propene insertion.89,90,147,200,274,276,277

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The possible dissociation of the monomer coordinated with the wrong enantioface can lead back to the propene-free intermediate or, directly, to the propenebonded intermediate with the right enantioface (through a ligand substitution reaction). For the sake of simplicity, minimum energy pathways that according to the calculations are expected to be similar, have been assumed to be identical, independently of the symmetry (stereoselectivity) of the catalyst. However, the plots for the syndiospecific (Figure 45B) and isospecific (Figure 45C) models are different since, as previously discussed, the enantioselectivities for primary and secondary insertions are in favor of the same or opposite monomer enantiofaces, respectively. In this framework, the lower regioselectivity of class II C2-symmetric isospecific metallocenes can be rationalized by assuming that the activation energy for rotation of propene coordinated with the “wrong” enantioface, between the orientations suitable for primary and secondary insertions (schematically shown by dotted lines in Figure 45), is in general lower than (or comparable to) the activation energy for a primary insertion leading to a stereoerror91,155 (see Scheme 41). On the other side, highly substituted C2-symmetric ansa-metallocenes, such as rac-C2H4(4,7-Me2-1-Ind)2ZrCl2, rac-C2H4(4,7-Me2-H4-1-Ind)2ZrCl2, and rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2, show relevant amounts of secondary insertions that molecular modeling indicates are due to a direct interaction between a correct primary coordinated propene and the substituent on C(4) of indene, the same substituent that is the key to high isospecificity in these systems. Hence stereoselectivity and regioselectivity are strictly related. As a rule-of-thumb, for zirconocenes of the general formula shown in Chart 7, the more stereoselective the catalyst, the less regioselective it is. For syndiospecific model complexes, since their enantioselectivity is in favor of the same monomer enantioface both for primary and secondary insertions, when coordination of the monomer occurs with the wrong enantioface for primary insertion, the most probable event is the dissociation of the monomer. With lower probability, primary insertion of the wrong enantioface is also possible, thus introducing a stereoirregularity in the polymer chain. Secondary insertions are expected to be essentially absent (see the high energy of situations d and e in Figure 45B). With the assumption of a low-energy barrier for rotation of the propene molecule between the orientations suitable for primary and secondary insertions, regioselectivity would be simply determined by the differences between the activation energies for secondary and primary insertions of the more suitable enantioface (∆E‡sec,r - ∆E‡pri,r) (and independent of the energy barrier for the monomer coordination). Moreover, regioselectivity is expected to be high and similar to that of the corresponding aspecific catalytic complex.91 For isospecific model complexes, since their enantioselectivity is in favor of opposite monomer enantiofaces for primary and secondary insertions, when the coordination of the monomer with the enantioface



Scheme 41. Influence of Nonbonded Interactions on Regiochemistry

unsuitable for the primary insertion occurs, besides the dissociation of the coordinated monomer and besides a low probability primary insertion (generating the stereoirregularities), also a low probability secondary insertion (generating the regioirregularities) would be possible. This is due to the fact that the barrier for the dissociation of the coordinated monomer is not expected to be negligible with respect to the activation energy for the secondary insertion. Hence, for these isospecific model complexes, the amount of regioirregularities in the polymer chains would be not determined (as for the cases of aspecific and syndiospecific model complexes) by the differences between the activation energies for the secondary and primary insertions. Instead, it would be related to the difference between the activation energies for the dissociation of the monomer (coordinated with the wrong enantioface) and the activation energy for its secondary insertion (∆E‡sec,w ∆E‡diss,w).

D. Influence of Monomer Concentration The important influence of propene concentration and polymerization temperature on the regioregularity and end group structure of metallocene i-PP have been realized only recently, thanks to detailed 1H and 13C NMR analysis of the polymers made with some prototypical zirconocenes. No data are available on the corresponding hafnocenes or titanocenes. With the moderately isospecific rac-C2H4(1-Ind)2ZrCl2/ MAO catalyst, the total amount of secondary insertions does not depend on monomer concentration, while it shows a dependence on polymerization temperature. On the other hand, the chemical structure of the chain fragment generated by an isolated secondary unit does depend on both polymerization temperature and monomer concentration. Busico252 and Resconi232 have shown that 2,1 f 3,1 isomerization is a unimolecular process, as the ratio of [2,1]/[3,1] follows a simple first-order dependence on monomer concentration (see Scheme 42): the 2,1 units are more likely to isomerize into 3,1 propene units when the monomer concentration is lowered (or the polymerization temperature increased). The kinetics of isomerization have been described by eq 2, where sRp and skp are the rate and constant of insertion of a primary unit onto an active site with a secondary chain end, sRis and skis are the rate and constant of secondary chain end isomerization, and sC and sC‚M are the active centers having a secondary chain end and none or one coordinated monomer

Scheme 42

molecules, respectively, and are correlated by the equilibrium constant for monomer coordination, sK ) [sC‚M]/[sC][M]: s Rp skp[sC‚M] skpsK[M] [2,1] ) s s ) ) s [3,1] sRis kis[ C] kis

(2)

The monomer concentration dependence of the 2,1f3,1 isomerization in the case of the less regioselective rac-C2H4(4,7-Me2-1-Ind)2ZrCl2/MAO catalyst has been investigated by Resconi and co-workers.342 While for rac-C2H4(1-Ind)2ZrCl2/MAO at 50 °C in the system toluene/propene, both the 2,1e/2,1t ratio (e/t ≈ 2) and the overall percentage (≈0.6%) of regioinverted units including 3,1 remains approximately constant with [M];232 however, in the case of rac-C2H4(4,7-Me2-1-Ind)2ZrCl2/MAO the total amount of secondary units decreases distinctly with decreasing monomer concentration, from ca. 3% in liquid monomer to almost 1% at [M] f 0 (Figure 46). In this case,

Figure 46. Total secondary insertions, including end groups, versus propene concentration in i-PP samples from rac-C2H4(1-Ind)2ZrCl2/MAO (4) and rac-C2H4(4,7-Me2-1Ind)2ZrCl2/MAO (9) (50 °C, toluene).342


Figure 47. [2,1]/[3,1] ratio (determined by 13C NMR) versus propene concentration in i-PP samples from racC2H4(1-Ind)2ZrCl2/MAO (4) and rac-C2H4(4,7-Me2-1-Ind)2ZrCl2/MAO ([) (50 °C, toluene). For both (sKskp/skis)50°C ) 3.342

we can no longer assume that regioselectivity is independent from monomer concentration. However, the experimental [2,1]/[3,1] ratios for samples from rac-C2H4(1-Ind)2ZrCl2/MAO and racC2H4(4,7-Me2-1-Ind)2ZrCl2/MAO show the same linear dependence on propene concentration with a correlation parameter R ) 0.989, from which we obtain the same sKskp/skis ) 3 (Figure 47). Hence, despite their different regioselectivities and activities, the rate of isomerization for these two zirconocenes is the same. These results confirm that (i) both the fraction of 2,1 insertions and the rate of isomerization, relative to primary insertion, increase by increasing the bulkiness of the ligand, (ii) isomerization is a unimolecular process, with a zero-order dependence on monomer concentration, and (iii) differences in the ligands have no influence on the isomerization mechanism.

E. Influence of Polymerization Temperature As already discussed in the section dedicated to stereoregularity, besides the biscyclopentadienyl ligand structure and the concentration of propene, the polymerization temperature is another important source of variability in the microstructure of polypropenes obtained from ansa-zirconocenes. In liquid monomer, both the amount of secondary insertions and the rate of 2,1 f 3,1 isomerization increase with increasing Tp. For example, for rac-C2H4(1-Ind)2ZrCl2/ MAO and rac-C2H4(4,7-Me2-1-Ind)2ZrCl2/MAO, total 2,1 units (including end groups) increase, between 20 and 70 °C, from 0.4 to 0.7% and from 2.5 to 4.3%, respectively.342 The C1-symmetric Me2C(3-t-Bu-Cp)(9-Flu)ZrCl2 interestingly shows a near constancy of the 3,1 units (∼0.4%) with Tp.470

F. 2,1 f 3,1 Isomerization Mechanism As discussed above, based on kinetic evidence, the 2,1 f 3,1 isomerization reaction must be unimolecular. Possible mechanisms of 2,1 f 3,1 isomerization have been discussed by Soga,463 Chien,473 and Kaminsky474 and more recently by Prosenc and Brintzinger,215 but no conclusive evidence has been presented. Chien suggested that isomerization occurs by a two-step mechanism (that is, first β-H transfer from the methyl group of the last (secondary) inserted unit, rotation of the coordinated R-olefin end group from

Resconi et al.

secondary to primary, and then reinsertion into the metal-hydrogen bond. This mechanism is shown in detail in Scheme 43. The modeling work of Prosenc and Brintzinger on the Cp2ZrR+ site (R ) n-propyl, isobutyl) led them to conclude that a mechanism occurring through β-H transfer, olefin rotation about the Zr-H bond, and reinsertion would be the lowest in energy, and hence the most likely one.215 Their modeling investigated the isomerization of an n-propyl or an isobutyl alkyl group to isopropyl or tert-butyl groups. The first step, β-hydrogen transfer to the metal, is the one with the higher activation barrier, about 10 kcal/mol for both alkyl groups. The second step, rotation of the olefin, was calculated to be of very low energy, about 1 kcal/ mol, in agreement with similar conclusions of other authors.56,146,176 The last step to complete the isomerization reaction requires insertion of the rotated olefin into the Zr-H bond, a step of usually negligible barrier.152-154,215,475 Very similar conclusions have been obtained by Rytter, Ystenes, and co-workers, who studied the same isomerization reaction of the longer n-butyl group in the Cp2ZrC4H9+ and (Me5Cp)2ZrC4H9+ systems.475 Moreover, they found that the olefin rotation step was not hampered by steric pressure also with the bulkier Me5Cp ligands. Prosenc and Brintzinger also discussed two other possible mechanisms, that, is a concerted 1,2-H/2,1Zr shift (mechanism II, shown in Scheme 44), in which the Zr center at the CR and one of the H atoms at the Cβ exchange their positions simultaneously, and a third one involving a flip-over of the coordinated olefin, after the usual β-H transfer step, via a transition state in which the flipping olefin is edgeon coordinated to the metal center through a double σ-CH coordination (mechanism III, shown in Scheme 45).215 They calculated a high activation barrier for a direct 1,2-hydrogen shift, roughly 40 kcal/mol, which rules out the direct 1,2-H shift as a viable mechanism, while the olefin flip costs about 7 kcal/mol from the ordinary π-coordination geometry.215

VIII. Kinetics Although Ziegler-Natta polymerization catalysts are extremely important and have been the subject of intense investigation for over 45 years, there is still a lot of controversy about the intimate nature of the active species and the mechanisms involved in this highly stereospecific chain growth process.444 At their appearance, stereoselective metallocenes were heralded as the first well-defined working model for heterogeneous Ziegler-Natta catalysts, and kinetic analysis was hence applied to this new class of catalysts aiming at a better understanding of the polymerization pathways. The more extensively metallocenes polymerizations were studied, the more evident the profound difference between the two catalyst families became and how much more complicated the polymerization pathways of the homogeneous catalysts could be. Kinetics describes reaction rates and their dependence from the reaction parameters. As we have extensively discussed in the previous sections, in



Scheme 43. Mechanism I, Showing Stereochemistry of Olefin Coordination

Scheme 44. Mechanism II

catalytic olefin polymerization several different reactions can occur at the active center. The relative values of the rate constants of these different possible reactions determine the microstructure of the obtained polymer under certain polymerization conditions. Kinetic analysis, hence, is one of the most powerful tools for the comprehension of reaction mechanisms.

Scheme 45. Mechanism III, Showing Stereochemistry of Olefin Coordination

Concerning activity, the propagation elementary step obeys the simple relationship: RP ) kP[C][M]. Under usual polymerization conditions, chain release and deactivation affect the number of active centers, hampering the correct evaluation of the propagation constant. To eliminate or reduce the effect of these side reactions two methods are used for kinetic investigation of the propagation step: living polymerization or stopped flow measurements. Living po-


lymerization conditions can be achieved by decreasing the polymerization temperature until chain termination and deactivation are so slow that propagation is the only mechanism taking place during the observation time, i.e., the duration of the experiment. This implies that the range of temperature in which is possible to have living polymerization is far from the typical range of polymerization temperatures. Propene living polymerization with the [t-BuNSiMe2Flu]TiMe2/B(C6F5)3 catalytic system at -50 °C was recently investigated to provide direct evidence of a second-order dependence of propagation rate on monomer concentration.476 In the case of stopped-flow technique, the reduction of the deactivation is achieved by shortening the time of the experiment, typically on the order of a fraction of a second. The stoppedflow approach was widely used in studying propene polymerization promoted by heterogeneous ZieglerNatta catalysts. Shiono et al.477 and Busico et al. 478 applied this technique to the kinetic investigation of ethene and propene polymerization with homogeneous catalytic systems. Busico investigated ethene and propene polymerization in the temperature range 20-60 °C with the homogeneous system rac-Me2Si(2-Me-Benz[e]ind)2ZrCl2/MAO; he obtained turnover frequencies of monomer insertion of 6.5 × 104 and 1.6 × 103 s-1 at 40 °C, respectively, for ethene and propene polymerization. In the case of polyethylene, the propagation rate measured under these short time of polymerization are 2 orders of magnitude higher than those reported for polymerization runs at higher reaction time. This can suggest that diffusion limitations occur at longer time. For polypropene, diffusion limitations seem to be less important, perhaps because of the lower rate of propagation or the higher solubility of the polymer in the reaction medium. Apart these specialistic approaches, in the following paragraphs the reported kinetic investigations are referred to experiments under more conventional polymerization conditions.

A. Activity versus Metal Several transition metals have been found to give active metallocene catalysts, all among groups 3 and 4 and the lanthanides and actinides. Almost always, Zr is the most active, although there are important exceptions to this rule. For example, the titanocenes C1-I-3-anti/syn are much more active than the corresponding zirconocenes.389 In most instances, both Hf and Ti give much less active catalysts, and often these are inactive. Sc and Y also have very low activities. The generally low activity of Ti can be attributed to its tendency to reduction, while the low activity of Hf complexes has been attributed to the higher strength of the Hf-C bond compared to the Zr-C bond.

B. Activity versus Catalyst/Cocatalyst Ratio Since the discovery of methylalumoxane as effective cocatalyst for the activation of metallocenes, many studies have been devoted to a better understanding of the nature of the active species and of

Resconi et al.

the role of MAO as cocatalyst. It is now generally accepted that MAO first alkylates metallocenes dichloride and then generates cationic species by abstracting and complexing the counterion.417,479 The cationic nature of the active species was clearly demonstrated by the results obtained by Jordan et al.,480 who showed that the zirconium complex [Cp2ZrCH3(THF)]+[BPh4]- polymerizes ethene in a polar solvent, such as methylene chloride, without any activator. Subsequently, many MAO-free cationic metallocene systems, able to polymerize ethene, propene, and higher R-olefins, even in nonpolar solvent, were developed.224,225,481,482 Several 1H and 13C NMR investigations483-485 as well as spectroscopic (UV/ visible)21,486-488 and conductometric studies487,488 were performed to obtain insight into the mechanism of active species formation. Depending on the Al/Zr molar ratio, the formation of monomeric or dimeric structures (in which the vacant site is stabilized by a µ-CH3 coordination) has been postulated, each with different activity, leading to different activity at different Al/Zr. Despite these extensive efforts in the study of the interactions between MAO and metallocenes, it is still unclear why a large excess of MAO is required to achieve the maximum activity. It is generally reported in the literature417,489,490 that activity increases by increasing the Al/Zr molar ratio up to an optimal value, after which a decrease of the polymerization rate is observed or, as reported in other papers,479,487,488,491 up to a plateau value. This behavior is common to many different catalytic systems either syndio- or iso- or aspecific and also to MAO-free catalysts.74,485 Fink et al.489 investigated ethene and propene homopolymerization by means of the Cs- (Me2C(Cp)(9-Flu)ZrCl2/MAO) and C2-symmetric (Me2Si(1-Ind)2ZrCl2/MAO) catalytic systems in toluene at different catalyst concentration and different Al/Zr molar ratios. They found a first-order dependence of the polymerization rate from [Zr] at constant monomer concentration and Al/Zr. By varying the Al/Zr molar ratio at constant [Zr] and [M], the activity of the Me2Si(1-Ind)2ZrCl2/MAO catalytic system shows a maximum at Al/Zr = 27 000 for ethene and Al/Zr = 11 000 for propene polymerization. In the case of the Me2C(Cp)(9-Flu)ZrCl2/MAO system, a similar behavior was observed with the maximum at Al/Zr = 5100 for ethene and Al/Zr = 1300 for propene polymerization. Furthermore, with the Cs-symmetric catalyst, the fraction of rmrr pentads and the molecular weights decrease with increasing Al/Zr ratio. On the contrary, PP stereoregularity and molecular weights remain constant in the case of the C2-symmetric catalyst. The increase of the rmrr pentad indicates a dependence of the rate of chain back-skip on the counterion, that is, complex reaction mechanisms which might be composed of several single steps and equilibria. At low Al/Zr molar ratio, MAO may not be sufficient to convert all the metallocene into active cationic catalyst. At optimum Al/Zr ratio the metallocene is converted to cationic complexes with weakly coordinating MAO-complexed counterions. At higher Al/Zr, the excess of MAO and olefin can compete for vacant coordination sites. The different effect of the MAO concentration on the


microstructure of the obtained polymer for the two catalysts is due, according to Fink, to the different ligand geometry. The Me2C(Cp)(9-Flu) ligand has a shorter bridge and therefore a larger angle between the planes of the π-ligands system than the Me2Si(1-Ind)2 ligand. Hence, in Me2C(Cp)(9-Flu) the Zr atom is less shielded by the ligand and the interaction between Zr and MAO component is more efficient. As a consequence of the tight contact ion pair, the maximum activity appears at much lower Al/Zr molar ratios both for ethene and propene than for Me2Si(1-Ind)2ZrCl2. In a further investigation423 using the Me2C(3-MeCp)(9-Flu)ZrCl2/MAO catalytic system for propene polymerization, the maximum activity vs Al/Zr was observed at a still lower ratio, Al/Zr = 500, suggesting that the shielding effect in this case is still lower due to the higher hindrance of the methyl group on the Cp ligand. Mu¨lhaupt479 investigated propene polymerization with the rac-Me2Si(2-Me-Benz[e]ind)2ZrCl2/MAO system in toluene and found a similar bell-shaped profile for activity vs cocatalyst concentration. In the same direction, a decrease of the molecular weight but no effect on stereospecificity was found. According to Mu¨lhaupt, at high [MAO], coordination of MAO instead of the olefin can lead to polymerizationinactive species that can undergo chain termination via β-H elimination, generating a different Rp/Rr ratio and hence affecting the molecular weight. He concluded that it is the overall MAO concentration that plays a crucial role in the activation of the metallocene and not the Al/Zr molar ratio. Considering that MAO is a gel rather than a true solution, the dilution of this component can affect the actual efficiency of the cocatalyst. Further evidence that the close contact of the cocatalyst to the catalyst is a key point comes from the supportation results: supported catalyst systems reach high activities at much lower MAO/ metallocene ratios compared to the homogeneous systems. Fink extended his investigation on the effect of the cocatalyst/catalyst ratio to the system Me2Si(1Ind)2ZrCl2/[Bu3NH][B(C6F5)4]74 for ethene and propene polymerization in comparison with the same MAO-activated catalyst. The plot of the activity vs [metallocene]/[ammonium borate] is a sigmoidal curve, indicating the existence of two or more successive equilibrium reactions. The plot of activity vs [ammonium borate]/[metallocene] in the range 0-3 presents a maximum as in the case of the MAO-activated system, but now the maximum corresponds to cocatalyst/catalyst ) 1. The polymers obtained with Me2Si(1-Ind)2ZrCl2 activated by either [Bu3NH][B(C6F5)4] or MAO show the same microstructure, indicating that the stereoselectivity of the metallocene catalysts does not depend on the nature of the cocatalyst and that the active species are similar in the two cases. This is a key observation, in accordance with the report of Ewen46 and our own experience. Nevertheless, the two catalyst systems differ considerably in the mechanism of formation of these active species. Similarly, Mu¨lhaupt et al. found for propene polymerization with rac-Me2Si(2-Me-Benz[e]ind)2ZrX2 (X ) Cl, Me)


that polypropene melting temperature and isotacticities are independent of the catalyst source (metallocenedichloride or dimethyl) and of the cocatalyst nature (MAO or cationic activators).492

C. Activity versus Time Most of the studies on olefin polymerization with metallocenes were performed under polymerization conditions in which deactivation occurs, which makes it difficult to determine the precise number of active centers and hampers a correct kinetic analysis. There are a few studies in the literature reporting the deactivation behavior of metallocene catalysts as a function of temperature. Both for ethene and propene homopolymerization typically the polymerization rate reaches its maximum immediately after the reaction is initiated. Sometimes an induction period is observed,493 and then the rate gradually decreases with time to a lower steady-state value.438,494-496 The decay profile depends mainly on the polymerization temperature. The time to reach the steady-state activity ranges between minutes (for temperature of 40-60 °C) to hours (for temperature of 0-20 °C). Mu¨lhaupt and Fischer have studied propene polymerization kinetics using the achiral, nonstereoselective (aspecific) Cp2ZrCl2/MAO catalyst in the temperature range 0-60 °C, in toluene at the same propene partial pressure (2 bar).496 The total catalyst productivity increases by increasing the polymerization temperature (taking into account the higher propene concentration at lower temperature). At all the temperatures the authors observe that the maximum catalyst activity is reached within few seconds; hence, they conclude that there is no induction period and that the activation of the catalytic complex is very fast. For temperatures below 40 °C they observe a slow decrease of activity over a period of several hours, and at higher temperature there is a very rapid initial decay followed by a second deactivation process. They propose a two-step deactivation process in which the active centers C are deactivated through a reversible intermediate I(1), followed by an irreversible process to form the inactive species I(2):

2 C h I(1) f I(2) The irreversible process is introduced at the higher temperatures to account for the second slow decay. On the basis of this model, both processes are secondorder with respect to the active sites. Rempel investigated the kinetics of propene polymerization with the stereoselective system racC2H4(1-Ind)2ZrCl2/MAO in a gas-phase reactor aiming at studying the effect of [Zr], [Al]/[Zr], and temperature on the catalyst activity.495 The authors report that, also in this case, the polymerization rate reaches its maximum in a short time and then gradually decreases with reaction time. They conclude that there is no induction period at the beginning of the polymerization. The conclusion of their study is that polymerization temperature is the most important factor influencing polymerization kinetics, because the catalyst is less stable at higher temperature and deactivates faster. Higher level of [Al]/[Zr]


enhances the catalyst activity but shows no effect on catalyst deactivation. To explain their experimental results, the authors propose the existence of two types of active centers, species A(1) and A(2). Species A(1) is highly active but unstable at elevated temperature, while species A(2) is very stable but has low activity. Species A(1) can be irreversibly transformed into species A(2). Vela Estrada and Hamielec497 as well as Chien and Wang498 investigated ethene polymerization again with the Cp2ZrCl2/MAO catalytic system. Although ethene polymerization is out of the scope of this review, Hamielec’s work is cited because he proposes a kinetic model for the deactivation that has been verified later also in the case of propene polymerization. The experimental data show that a higher deactivation takes place at a polymerization temperature of 50 °C with respect to 70 °C; moreover, always at 50 °C, GPC analysis shows a bimodal molecular weight distribution that the authors interpretate as the evidence of the existence of two catalyst site types. To justify these experimental findings they postulate a rapid initial formation of the active center type C(1) that in part are converted to a second type of active site C(2), which can deactivate through a bimolecular mechanism: M

C(1) f C(2) 98 D More recently, Rytter et al. investigated the polymerization of propene and the deactivation rates of Me2Si(1-Ind)2ZrCl2/MAO and Me2Si(2-Me-1Ind)2ZrCl2/MAO as a function of [Zr], temperature (in the Tp range from 40 to 130 °C), and pressure (in the range 1-2.5 bar).493 The authors attempt to describe the polymerization profiles of both catalysts using different models described in the literature and discuss also their validity. Differently from most previous investigations, Rytter states the existence of an activation period, i.e., a period of increasing polymerization rate before maximum activity is reached, followed by a period of deactivation. About the activation period, on the basis of their experimental findings, they are able to discard possible causes such as the slow alkylation of the metallocene by MAO (reported in ref 499). In fact, the catalyst is allowed to react with MAO before being injected into the reactor; moreover, increasing the aging time of the catalyst/MAO solution or the activation temperature of the same solution does not shorten or remove the activation period observed during the polymerization. The explanation proposed is a slow rate of insertion of the first monomer unit in the Zr-methyl bond of the alkylated metallocene. Concerning deactivation, the authors took into account the hypothesis developed in the literature of the formation of latent and permanently deactivated sites. They combine in a very complete kinetic model the activation step, the polymerization, the deactivation, and the dependence of the different rate constants upon the temperature. The experimental and calculated polymerization rate profiles of Me2Si(1-Ind)2ZrCl2/MAO are well in agreement over the entire temperature range (40-130 °C), whereas the polymerization rate of Me2Si(2-Me-1-

Resconi et al.

Ind)2ZrCl2/MAO is adequately described only over a limited temperature range. A possible deactivation mechanism is proposed: a reversible formation of latent deactivated sites, which is first-order with respect to propagating sites, and an irreversible formation of permanently deactivated sites, which is second-order with respect to propagating sites. A first-order formation of the permanently deactivated sites, rather than a second-order one, gives a better fit of the rate profiles of Me2Si(1-Ind)2ZrCl2/MAO at higher temperature. A different hypothesis on the nature of the latent or dormant sites can be found in the literature. Busico et al.252 estimated that more than 90% of the active sites are “trapped” in a dormant state by 2,1-propene insertion: this assumption requires that the deactivation rate should be monomer dependent. To verify this hypothesis Rytter et al. performed some experiments with interrupted monomer flow, and under these conditions the catalyst continues to deactivate, even when no polymerization occurs. That means that, at least for the catalysts and under the experimental conditions described by Rytter, monomer misinsertion cannot be the cause of rate decay, that is, the deactivation mechanism is monomer independent. Rytter observed that the rate of decay highly depends on the Zr concentration: at low Zr level the polymerization rate is quite constant versus polymerization time, and at high Zr level there is a fast decrease in polymerization rate during the first hour. Other studies show that the maximum activity increases by increasing catalyst concentration, but at the same time a more quick deactivation occurs.499 NMR studies484,485 have shown the existence of monomeric and dimeric metallocene/cocatalyst ion pairs in equilibrium, depending upon the catalyst concentration, the catalyst/ cocatalyst ratio, the nature of the cocatalyst, and the temperature. MAO too is observed to affect activity.500 Rytter observed that at high MAO concentration the maximum activity is lower but the polymerization rate is more stable than at low MAO concentration. It is possible that if deactivation is caused by two metallocene molecules reacting to form a nonactive species, MAO can compete in this reaction and prevent the formation of deactivated sites. After the kinetic study of propene polymerization with the aspecific Cp2ZrCl2/MAO system, Mu¨lhaupt widens the investigation to two ansa-zirconocenes (rac-C2H4(1-Ind)2ZrCl2 and rac-C2H4(H4-1-Ind)2ZrCl2) aiming at investigating the effect of the addition of Lewis acids and bases on the activity of the catalytic system.500 The kinetic profiles found in the case of the two stereoselective catalysts are similar to those reported for the Cp2ZrCl2/MAO system. In this paper he suggests that the reversible second-order deactivation may result from zirconocene dimerization or disproportionation and that “MAO may be involved in the dynamic equilibrium between neutral dormant sites and cationic active sites”.500 The addition of Lewis acids or bases can influence these equilibria, affecting catalyst productivity but not the steric control.


Chemical Reviews, 2000, Vol. 100, No. 4 1331 Scheme 46

D. Kinetic Models: Activity versus Monomer Concentration According to the Cossee mechanism, the two key steps in Ziegler-Natta polymerizations are monomer coordination and migratory insertion into the metalpolymer chain bond (Scheme 10). In analogy to heterogeneous Ziegler-Natta catalysis, a first-order reaction rate with respect to monomer concentration is generally assumed also for metallocene-based catalysts, that is,

Rp ) kp[C][M]

(3)

Equation 3, however, requires that monomer coordination is the rate-determining step. However, there are many observations which are very difficult to explain by a simple single-site Cossee mechanism, such as a reaction rate order higher than one reported for propene,113,230,232,417,428,441,467,501 ethene,501 styrene,502 and diene503,504 polymerizations. To fit these observations, some authors113,501 have proposed that the active center can coordinate two monomer molecules. Ystenes, following a different approach, proposed a different model able to explain a reaction rate order higher than 1.203,505 This mechanism (dubbed the “trigger mechanism”) involves a transition state in which the insertion of a coordinated monomer is triggered by a second monomer. The main assumptions of this model are (i) the active site is never vacant, as a new monomer will coordinate to the site at the same time the previously coordinated monomer is inserted; (ii) the insertion will not proceed or will proceed more slowly in the absence of the new monomer unit; (iii) in the transition state two monomer units interact with each other and with the metal atom. On the basis of these assumptions, the propagation step must be first-order with respect to monomer concentration, but the number of active centers may be dependent on the monomer concentration, as a monomer unit is needed for the formation of an active center. Hence the overall polymerization rate can be anything between first- and second-order with respect to monomer concentration. This effect may disappear in the case of saturation (all the active centers are propagating) or of monomer diffusion control. The existence of physical limitations was one of the most typical explanations invoked for the deviation from a linear dependence of the activity on monomer concentration. This kind of indirect reason, such as mass transfer or heat transfer, for propene polymerizations has been ruled out by careful experiments by Mu¨lhaupt and co-workers,467 who pointed out that “equilibria involving the active species are responsible for this effect” and that “propene might be involved in an equilibrium between dormant and active sites”. Busico and co-workers,250,252 investigating the isospecific rac-C2H4(1-Ind)2ZrCl2/MAO system for propene polymerization, reported an effect of monomer concentration not only on the activity but also on the regioselectivity of the catalytic system. It is stated that it is the occurrence of regioirregular 2,1 insertions that slows down chain propagation. The active centers bearing a secondary growing chain would be

trapped in a dormant state, because of higher steric hindrance to following primary insertion, that is, skp , pkp. It is difficult to consider this hypothesis as a general one, because a reaction order >1 is observed also for the much more regioselective Me2C(Cp)(Flu)ZrCl2 catalyst112,113,417,428 and for the rac-C2H4(1-Ind)2ZrCl2/MAO catalyst, for which the total amount of secondary units appears to be constant with propene concentration. Resconi et al.232,418 proposed a model in which at the steady state the active center can be in two active states that differ for their monomer insertion or coordination rates. The main assumptions of this model are (i) the two states differ for their propagation rate constants (a faster propagating state Cfast and a slower one Cslow), (ii) the two states are interconverting and their interconversion does not involve the monomer, and (iii) monomer insertion transforms a slow center into a fast one (see Scheme 46). According to this kinetic scheme, the propagation rate law results:

Rp [C]

(

kffs +

)

)

kp,fastksff [M] + kp,fast[M]2 kp,slow kffs + ksff + [M] kp,slow

(

)

(4)

A reaction rate order higher than 1 is due to the decrease of the concentration of the slower state as the monomer increases. The ability of this equation to reproduce the trends of the experimental activity depends on the relative value of the kinetic rate constants. It is worth noting that eq 4 converts to the first reaction order for ksff g kffs, or kp,slow[M] . kffs, ksff, for which Rp ≈ kp,fast[C][M]. On the other hand, eq 4 approaches the second reaction order, Rp ≈ kp,fast[C][M]2, when kp,fast [M] . kffs . kp,slow[M] . ksff. Hence, eq 4 corresponds to a reaction order higher than 1 on monomer concentration for kp,fast[M] > kffs > kp,slow[M] > ksff, that is, when the slower state of the catalytic center is of lower energy with respect to the faster one, and the interconversion rate between the fast and the slow state is intermediate between the fast and the slow chain propagation rates. Figure 48 shows three series of experimental activity data for propene polymerization, obtained with zirconocenes of three different symmetries, and their best fit to eq 4. A possible set of kinetic constants relative to propene polymerization with the two catalytic systems Me2Si(2-Me-Benz[e]ind)2ZrCl2/MAO and Me2C(Cp)(9-Flu)ZrCl2/MAO were derived on the basis of the suggested model. The kinetic scheme proposed requires two different alkene-free states of the cata-


Resconi et al.

to the active species C (as an equilibrium with the free monomer) and the insertion step,

(C)n + M h (C)nM f (C)n+1 for which the overall reaction rate law is

Rp ) A[M]/(1 + B[M])

(5)

In the case of a double monomer coordination, the model requires three steps:

(Cn) + M h (C)nM (C)nM + M h (C)nM2 f (C)n+1M In this case the rate law is

Rp ) A[M]2/(1 + B[M] + C[M]2) Figure 48. Experimental activity versus [M] data for three stereoselective zirconocene/MAO catalysts, and their best fit to eq 4: A, Me2Si(2-Me-Benz[e]ind)2ZrCl2/MAO;467 B, Me2C(Cp)(Flu)ZrCl2/MAO;417 C, Me2C(3-PhCp)(Flu)ZrCl2/ MAO.441

lyst active center: a higher energy state faster in monomer insertion and a lower energy state slower in monomer insertion. The authors give a possible interpretation of the nature of the two states, suggesting that they can differ in the conformation of the growing polymer chain. Several theoretical calculations have in fact indicated that the kinetic product of monomer insertion is a γ-hydrogen agostic intermediate, while the resting state has the β-hydrogen agostic interaction. Other possibilities can be proposed to explain the nature of the slow state of an active polymerization center, which can be different for different catalytic systems, monomers, and experimental conditions. For instance, Richardson assumes an ion-pairing equilibrium between a metal-methyl cationic species and the counterion, which is in competition with initiation, but of course not with propagation.457 This assumption seems questionable especially for the common case of prevailing chain termination by β-hydrogen transfer to the monomer. In fact, in the latter case, an alkyl group with at least two carbon atoms (hence a steric requirement similar to that of the growing chain) is already coordinated to the metal center at the initiation step. The same effect could be generated by a coordinated AlR3 or neutral zirconocene species.506-508 It is worth noting that, when β-transfer events are not negligible with respect to propagation, the slower state could correspond to initiation centers deriving from some kind of chain release that does not involve the monomer (e.g., a β-hydrogen transfer to the metal rather than to a coordinated monomer molecule). The occurrence of a nonlinear propagation rate law is discussed in ref 509. Marques and co-workers considered the two limiting situations, a single monomer coordination and a double monomer coordination, and their combination.501,510,511 In the first case a two-step kinetic scheme is reported: the coordination of the monomer

(6)

They consider also a more general situation in which both singly and doubly coordinated complexes can exist, and both of them can be active in polymerization. The more complex rate law is

Rp ) (A[M] + B[M]2)/(1 + C[M] + D[M]2) (7) The last equation was used to fit the experimental trend of activity versus monomer concentration with three different catalytic systems: the [2-(N,N-dimethyl)aminoethyl]cyclopentadienyl-TiCl3/MAO complex for ethene and propene polymerization, the racC2H4(1-Ind)2ZrCl2/MAO system, and the two analogues, Me2Si(Benz[e]ind)2ZrCl2/MAO and Me2Si(2Me-Benz[e]ind)2ZrCl2/MAO, for propene polymerization. In all three equations A, B, C, and D are apparent constants which are a combination of the real kinetic constants. The number of parameters that can be changed to fit the experimental curves is so high that almost every possible kind of curve can easily be reproduced. In the absence of a straight correlation between the value of the parameters and their physical meaning, the fitting of experimental results cannot prove the mechanism. Recently Brintzinger et al.512 reconsidered the consistency of the trigger mechanism. Although molecular model considerations would suggest that for steric reasons is impossible to have two monomer units simultaneously coordinated to the metal, they evaluate the influence of a second propene molecule on the energy reaction path for the insertion of the propene ligand into the Zr-C bond. The calculations were made under certain assumptions: the [(C5H5)2Zr-Ethyl(propene)]+ cation was chosen as a good compromise between computational time and a reasonable reliability as a model for the Zr-polymeryl active species. Moreover, the calculations are based on the assumption of a cationic species in vacuo rather than in a condensed medium and that the olefin insertion is the rate-determining step of the overall kinetics of polymerization. Two possible transition states are proposed, which differ for the presence in the second one of a propene unit approaching the reaction complex. The proximity of a second monomer unit decreases the activation energy for the insertion for the latter transition state. An interest-


ing feature of the proposed reaction sequence is the circumstance that the final insertion product has its next olefin already in place for the following insertion. The reaction rate law based on the mechanism proposed is

Rp ) [C]tot(A[M]2 + B[M])/([M]2 + C[M] + D) (8) Equation 8 predicts a reaction rate order between 1 and 2. The fitting of the turnover frequencies data for the system Me2Si(2-Me-Benz[e]ind)2ZrCl2/MAO with eq 8 is adequate only for certain reciprocal values of the parameters, which turned out to be inconsistent with the kinetic scheme; hence, additional hypothesis and calculations will have to be worked out. Recently Kaminsky et al. reported the results of the investigation of propene polymerization with C1symmetric zirconocenes.441 The three catalytic systems investigated, Me2C(3-PhCp)(Flu)ZrCl2, Me2C(3CyHexCp)(Flu)ZrCl2, and (PhMe3PenFlu)ZrCl2 (C1I-17), all show a nonlinear dependence of activity on [M], with reaction order between 1.4 and 1.5. This finding confirms the report of Fink on the hemiisospecific zirconocene C1-I-6.417 Hence, a reaction order on monomer concentration higher than 1 is a common feature for all stereoselective metallocenes. The exception to this rule is offered by the aspecific C2v-symmetric catalysts, like Cp2ZrCl2496 and Me2Si(9-Flu)2ZrCl2,123 that have activities that are first order in monomer concentration. At present, no definite explanation for this difference has been provided. In light of the results discussed above, it appears that literature reports on the effect of polymerization conditions (such as temperature, Al/Zr ratio, cocatalyst, and solvent type) on chiral metallocene performance (not only activity but also stereo- and regioregularity, propagation/transfer rates), unless obtained in liquid propene, should be taken with some precautions.

E. Activity versus Temperature Studies of temperature effects on propene polymerization of a number of metallocene catalyst systems show that the stereoregularity, streospecificity, and molecular weight of polypropene decrease with increasing polymerization temperature, whereas the polymerization activity increases.50,252,438,467,472,493,499 As discussed in the previous paragraphs, using experimental data of polymerization rate obtained at different temperature and different monomer concentration to calculate the activation energy of the propagation can alter the value of the true ∆E‡ by neglecting the influence of monomer concentration on activity. Moreover, deactivation has different rates at different temperature. For these reasons data of activation energy for the same catalytic system obtained from polymerization runs performed under different conditions can be very different. Again, it must be stressed that, when comparing the polymerization performance of different zirconocene catalysts, the experiments must be performed under high and identical monomer concentrations, and prefer-


ably in liquid propene, to minimize the extent of chain-end epimerization and other parasitic reactions (see sections V and VII). One of the earliest papers on kinetic investigation305 reported the study of propene polymerization with C2-I-1/MAO in a bubble reactor in the temperature range 15-65 °C. Kaminsky found that the activity has a linear dependence on monomer concentration at 35 °C in the propene concentration range 2-5 mol/L. Below 2 mol/L, the observed deviation from linearity is explained as due to masstransfer control. In the same range the molecular weight also increases linearly by increasing monomer concentration. A first-order propagation rate was assumed and the authors calculate the activation energy for the polymerization process by plotting the logarithm of the propagation kinetic constant versus 1/T. The diagram shows an abrupt deviation from linearity for temperature above 45 °C. Only the linear part of the diagram was used to determine the activation energy that resulted, 7.6 kcal/mol. Later Mu¨lhaupt et al.467 investigated the influence of polymerization temperature on activity for propene polymerization with the two isospecific catalysts, racMe2Si(Benz[e]ind)2ZrCl2 (C2-I-24) and rac-Me2Si(2Me-Benz[e]ind)2ZrCl2 (C2-I-25). Both catalysts show a dependence of activity on propene concentration of the same rate order of 1.7 and the same activity/time profile. Polymerization runs at 20, 40, and 60 °C at a total pressure of 2 bar were performed for both catalysts. rac-Me2Si(2-Me-Benz[e]ind)2ZrCl2 shows a very strong increase of activity with temperature, becoming faster than rac-Me2Si(Benz[e]ind)2ZrCl2 at 60 °C. This behavior agrees with Spaleck’s observation that catalyst activity for the 2-methyl derivative is higher than that of the nonsubstituted catalyst at 70 °C in liquid propene.320,335 The maximum activity observed was used for the Arrhenius plot, giving a ∆E‡ ) 6.7 kcal/mol for C2-I-24 and ∆E‡ ) 12.9 kcal/ mol for the 2-methyl derivative. The activation energy for C2-I-25 is surprisingly higher in comparison with the values for the C2-I-24 catalyst and Cp2ZrCl2. Rytter investigated the same two catalytic systems,493 but to evaluate the activation energy, he used the averaged activity, determined by the amount of polymer produced during 1 h of polymerization. The experimental polymerization temperature range is wider (40-130 °C) with respect to that reported by Mu¨lhaupt. Under this scenario the overall as well the maximum activity of C2-I-25 is always higher than that of C2-I-24. The activation energies for propagation calculated by Rytter are ∆E‡ ) 7.6 kcal/ mol for C2-I-24 and ∆E‡ ) 8.8 kcal/mol for C2-I-25. Recently Resconi et al.50,472 investigated the polymerization of propene with several C2-symmetric zirconocene/MAO catalysts in liquid monomer and in the temperature range 20-70 °C. The catalytic systems investigated are C2-I-1, C2-I-9, C2-I-31, C2I-34, C2-I-35, and C2-I-36. The plot of ln(A/[M]) versus 1/Tp gives the apparent activation energy values of the polymerization process, which are 11.4, 13.9, and 15 kcal/mol for the catalysts C2-I-1, C2-I-9, C2-I-31, respectively. For C2-I-34-36, a linear correlation in all the Tp range could not be obtained, due to reactor


fouling and, possibly, partial catalyst deactivation at the higher temperature. The same has been observed, for two C1-symmetric catalysts, by Kaminsky.441 The value of activation energy ) 11.4 kcal/mol found for C2-I-1/MAO is similar to the reported activation energy for propene polymerization with heterogeneous catalysts,245 which is in the range 10-13 kcal/ mol and significantly higher than the value of 7.6 kcal/mol reported by Kaminsky under nonconstant monomer concentration conditions.305

F. Activity versus Solvent As previously described and generally accepted, the active species in the polymerization of olefins with metallocene systems is an ion pair. It is clear that, depending on the polarity of the solvent, the strength of association of the ion pair can change, and hence, an effect on the activity of the catalytic system can be expected. There are several papers dealing with the solvent effect on the polymerization activity in polar solvent. In 1989 Oliva et al.502 reported that, by polymerizing propene with the Cp2TiCl2/Al(CH3)3 /Al(CH3)2F system in CH2Cl2, a conversion 100 times larger than in toluene was found. Several polymerization runs were performed at different compositions of the polymerization medium. Despite the strong effect on the activity, the stereochemical structure of the polymers obtained is quite the same in every case. The larger conversion observed in CH2Cl2 may be due to a larger amount of dissociated cationic species in a polymerization medium with a higher dielectric constant. The investigation was extended to an isospecific catalytic system (rac-C2H4(H4-1-Ind)2ZrCl2/Al(CH3)3/Al(CH3)2F for propene polymerization. Changing the solvent from toluene to methylene chloride yielded a 10-fold increase of the polymerization rate. Both the titanocene and the zirconocene systems showed the same behavior in the different polymerization media. Later Fink et al.74,489,513,514 studied propene polymerization with the syndiospecific Me2C(Cp)(9-Flu)ZrCl2/MAO and the isospecific Me2Si(1-Ind)2ZrCl2/ MAO systems in toluene/methylene chloride solvent mixtures. In the case of the Cs-symmetric Me2C(Cp)(9-Flu)ZrCl2, increasing the dielectric constant of the solvent mixture increases the polymerization rate linearly, and in nearly pure CH2Cl2 a polymerization rate higher by a factor 6 than in pure toluene is observed. In the same range of toluene/methylene chloride composition, the rrrr pentads almost linearly decrease from 89% in pure toluene to 42% in pure CH2Cl2, and consequently, the melting point of the polymer shifts from 143.7 to 47.4 °C. In the case of the isotactic Me2Si(1-Ind)2ZrCl2/MAO, a similar enhancement of activity by increasing the polarity of the solvent is observed, but the stereoselectivity remains constant over the entire range of compositions. Hence, the stereospecificity of these catalysts is connected with the existence of a polarized ZrCl-Al complex (in toluene) or of a tight ion pair with a stereoregulating role of the counterion. The loss of stereoselectivity in polar solvents is caused by an isomerization of the solvent-separated zirconocene species via migration of the growing chain before the

Resconi et al.

next monomer insertion. But in the case of the C2symmetric Me2Si(1-Ind)2ZrCl2/MAO system, the migration of the growing polymer chain causes no isomerization because the sites are identical by virtue of the C2 symmetry. More recently Deffieux et al.491 studied the polymerization of 1-hexene with C2-I-1/ MAO system in solvents of different polarity (toluene, methylene chloride, and heptane). Polymerization kinetics was monitored by dilatometry. Also in this case a strong enhancement of the polymerization rate is observed in pure CH2Cl2 compared to that measured in pure toluene. The use of methylene chloride allows one to reduce the amount of MAO by a factor of 20 to reach the plateau of maximum activity. No effect on the stereoselectivity of the catalyst is observed. Again the increase of the activity is explained by an easier ionic dissociation of the Zr-X bond (X ) Cl or CH3), which increases the population of the cationic active species and to the low-coordinating power of the CH2Cl2 molecule. In this study it is reported that a preactivation of the catalyst in methylene chloride allows retention of the high activity also if the following step of polymerization is performed in toluene or heptane. Since the amount of CH2Cl2 used in the preactivation step is only about 5% of the total volume of polymerization, the activation effect cannot be interpreted as due to an increase of the polarity of the polymerization medium. The polymerization rate observed in toluene, after catalyst preformation in CH2Cl2, is very close to the one found in pure methylene chloride. A strong improvement is found also for the polymerization in heptane by using the preactivation in polar solvents. This suggest that the active species are long-lived and, once formed, their concentration is quite insensitive to the nature of the polymerization medium. A possible explanation is that during the formation of the ionized metallocenium species assisted by MAO the chloride anion can be trapped by the cocatalyst and hence would be no more able to regenerate the starting inactive covalent species. A similar behavior was found by comparing ethene polymerization rate with the same catalytic system with and without preactivation in CH2Cl2.

G. Molecular Weight The molecular weight and molecular weight distribution are among the most important properties of a polymer, and this is true also for polymers that can have stereoregularity like polypropene, since a practical application can be found or foreseen for any polypropene tacticity, provided it has the right molecular weight. This means that molecular weights must fit both performance and processability requirements, and for the most part this means viscosity average molecular weights in the range 40 000-300 000. The molecular weight of a polyolefin (here defined as the average degree of polymerization, P h n) made with single-center metallocene catalysts (which operate by coordination polyinsertion mechanism) is given by


P hn )


∑Rp ∑Rr

That is, in terms of reaction rates, the molecular weight of polyolefins is given by the ratio between the overall rate of propagation (Rp) and the sum of all rates of chain release (Rr) reactions: this means that the molecular weight is dependent on the type of catalyst and the kinetics of the process, that is, the polymerization conditions (polymerization temperature, monomer concentration, catalyst/cocatalyst ratio). Hence, understanding the details of the mechanisms of chain release reactions is the key to molecular weight control in metallocene-catalyzed olefin polymerization. Here, chain release reactions (usually referred to as termination or transfer reactions) are all those steps that cause release of the polymer chain from the active catalyst, with the formation of a new initiating species (see section III.F). Molecular weight measurements are far less sensitive to variables which are by their nature most affected by experimental error, such as catalyst amount or catalyst/monomer purity, and, more important, on the number of active centers, but are highly sensitive to the catalyst structure, monomer concentration, and polymerization temperature. Hence, reliable molecular weights can give much information on the nature of the active sites. Studies of this kind have been reported by Mu¨lhaupt and Brintzinger467 on catalysts C2-I-24 and 25, by Kaminsky and Werner441 on three C1-symmetric zirconocenes, and by Resconi and co-workers,50,229,230,232,472 who investigated the dependence of molecular weight on the type of ligand, the polymerization temperature, and the concentration of propene for the C2-symmetric zirconocenes C2-I-1, C2-I-9, C2-18, C2-33, C2-35, and C2-36. The molecular masses of polypropenes obtained with metallocenes are clearly dependent on several factors. Among the most important substitution patterns are 2-alkyl and 3-alkyl substitutions. Methyl substituents on position 2 of the C2-symmetric skeleton have been shown to increase the molecular weights of the produced polymers considerably. A molecular modeling rationalization of this behavior has been proposed by Cavallo and Guerra.154 Approximated transition state geometries for the β-hydrogen transfer to monomer with the Me2Si(Benz[e]ind)2 ligand without and with a methyl group in position 2, are reported in Figure 49, parts A and B, respectively. The short distances between the CH2 groups of the growing chain and of the monomer with the methyl groups of the substitued Me2Si(Benz[e]ind)2 ligand suggest that a repulsive interaction between the substituted ligand with the growing chain and monomer are present. This destabilizing interaction is clearly absent when the unsubstitued Me2Si(Benz[e]ind)2 ligand is considered. In agreement with the results discussed in section IV.A, similar calculations performed on coordination intermediates for the propagation reaction indicated that the methyl groups in position 2 do not destabilize the insertion reaction.

Figure 49. Approximated transition state geometries for the β-hydrogen transfer to the monomer reaction on the systems based on the Me2Si(Benz[e]ind)2Zr ligand (part A) and its 2-methyl-substituted derivative (part B). The growing-chain is on the left, while the propene monomer is on the right. Short distances in Å. Adapted from ref 154.

Figure 50. Transition state geometries for the insertion reaction of ethene into the Ti-ethyl σ-bond, on the left, and for the β-hydrogen transfer to the monomer reaction, on the right. The metal ligands, not shown for clarity, are Cp rings.175

Hence, the only net effect is that methyl groups selectively destabilize the β-hydrogen transfer to monomer reaction, resulting in polymers with higher molecular weights. This behavior is due to the very different geometries assumed by the transition states of the propagation reaction and of the β-hydrogen transfer to monomer. The first transition state assumes a compact four-center geometry, and the angle spanned by the reacting atoms is roughly 90°. On the contrary, the latter transition state corresponds to a bulkier six-center geometry, and the angle spanned by the reacting atoms is roughly 140° (Figure 50). Due to these geometrical differences, the β-hydrogen transfer to monomer reaction is much more space demanding and is easily destabilized by the steric pressure of substituents on the ligand, whereas the propagation reaction can smoothly occur also in smaller


Resconi et al.

Table 18. Summary of Kinetic Data for Selected Isospecific, ansa-Bisindenyl Zirconocenes rac-zirconocene

∆∆E‡r (kcal/mol)

ref

Me2C(1-Ind)2ZrCl2 C2H4(1-Ind)2ZrCl2 C2H4(4,7-Me2-1-Ind)2ZrCl2 C2H4(3-Me-1-Ind)2ZrCl2 Me2C(3-Me3Si-1-Ind)2ZrCl2 Me2C(3-t-Bu-1-Ind)2ZrCl2 H2C(3-t-Bu-1-Ind)2ZrCl2

1.9 ( 0.1 3.9 ( 0.4 1.6 ( 0.2 4.1 ( 0.8 4.1 ( 0.3 10.7 ( 0.4 11.2 ( 0.5

50 50 50 50 50 50 324

reactive pockets. This feature is relevant not only for the C2-symmetric group 4 metallocenes but has been shown to be extremely effective also with ethene polymerization catalysts based on Ni141,515,516 and probably is also very relevant for the heterogeneous catalysts as well, due to the different coordination (octahedral) around the Ti atom, which clearly reduces the space available for the bulkier β-hydrogen transfer to monomer reaction.213 On the other side, 3-alkyl substitution on bisindenyl systems has a strong influence on increasing the energy of β-H transfers, but for this class of metallocenes then β-methyl transfer becomes competitive. The net result is that these catalysts produce high molecular weight PP at the highest monomer concentrations but low molecular weights similar to the unsubstituted systems at the lower [M]. The ln(P h n) versus 1/Tp plots give the overall activation energy barrier for chain release. The ∆∆E‡r values for a series of C2-symmetric zirconocenes are reported in Table 18. The higher energy barrier to transfer measured for C2-I-35/MAO and C2-I-36/MAO (10.7 and 11.2 kcal/mol, respectively) and the lower ones obtained from catalysts C2-I-1, C2-I-9, C2-I-31, and C2-I-33 have been attributed to the higher conformational freedom of the growing chain or the longer, more flexible C(3)-Si bond in C2-I-33. The analysis of the influence of monomer concentration on molecular weight is complicated by the competition between so many different chain release reactions, by the presence of two interconverting catalyst states, and by the onset of growing chain end isomerization reactions. Such an analysis is best done from end group structure analysis by 1H NMR, but it requires of course relatively low molecular weight polymers, to be able to quantify the end groups. Such an analysis has been carried out so far on only a few systems (see Figure 51).131,230,231

IX. Influence of Hydrogen Molecular hydrogen is used to regulate the molecular weight of polyolefins in both heterogeneous247,517and metallocene catalysts.83,248,249,252,467,518,519 Hydrogen response in metallocene-catalyzed propene polymerization seems to be wide-ranging and reflects the strong dependence of the performance of metallocene catalysts on both their π-ligands structure and on the polymerization conditions. Addition of molecular hydrogen produces much different levels of molecular weight depression depending on the hydrogen level, the concentration of the monomer, the type of catalyst, and the polymerization temperature. In addition, hydrogen often shows an activat-

Figure 51. Average degree of polymerization (by 1H NMR) as a function of [M]: I, i-PP from C2-I-35/MAO; II, i-PP from C2-I-1/MAO; III, i-PP from C2-I-18/MAO. Tp ) 50 °C. Scheme 47

ing effect for both families of catalysts. Because of these two features, hydrogen has been often used for mechanistic studies. For example, high hydrogen pressures have been used to produce propene oligomers (in the so-called hydrooligomerization reaction83-86,250,251,520) in order to determine the regiochemistry and stereochemistry of initiation and propagation. Pino applied the hydrooligomerization and deuteriooligomerization reactions, catalyzed by (-)-(R,R)-C2H4(H4-1-Ind)2ZrMe2/MAO, to propene and other 1-olefins. By analyzing the structures of the propene oligomers and measuring their optical rotations, he was able to show that monomer insertion is largely predominantly primary, that after an occasional 2,1 insertion chain growth is terminated by hydrogen with formation of the n-butyl end group, and that the (R,R) catalyst enantiomer preferentially selects the re monomer enantioface.83 This experimental observation enabled him to confirm the validity of Corradini’s model, that is, to conclude that enantioface discrimination in chiral ansa-metallocenes arises from the steric interaction of the monomer with the growing chain in its chiral orientation, which in turn is determined by the chirality of the catalytic complex (see section III). Activation can be moderate or quite substantial (up to 10-fold increase in catalyst productivity). The most likely mechanism of hydrogenolysis is the direct insertion of H2 into the metal-carbon bond (Scheme 47).103,521-526 However, since in a few instances H2 (especially at the highest concentrations) has been shown to decrease catalyst activity,124,125 other mechanisms could be possible, for example hydrogenolysis of one Mt-Cp bond.523,525,527 The structures of saturated PP end groups that can be produced in the presence of hydrogen are shown in Scheme 48. Two hypothesis have been described to explain the activating effect of hydrogen in propene polymerization with Ziegler-Natta catalysts: hydrogenation of a “dormant” secondary growing chain,248-252,518 or reactivation of a “torpid” allyl zirconocene.144,225,230,453,454,458


Tsutsui and Kashiwa described the influence of hydrogen on the catalytic efficiency of rac-C2H4(1Ind)2ZrCl2/MAO in toluene at 30 °C and low propene concentration.248,249 On the basis of the strong activating effect of hydrogen on this catalyst, the presence of n-butyl end groups, and the disappearance of regioerrors, they concluded that the activation is due to hydrogenolysis of a secondary growing chain, which must then have a lower kp compared to a primary growing chain, that is, skp < pkp. The same effect has been observed in 1-butene polymerization.518 Analysis of the hydrogen effect on the more stereoselective zirconocenes rac-Me2Si(Benz[e]ind)2ZrCl2 and rac-Me2Si(2-Me-Benz[e]ind)2ZrCl2467 showed a limited catalyst activation (38 and 17% respectively, 0.35 bar H2, 40 °C in toluene with 1.9 bar propene), a reduction of internal secondary units with formation of n-Bu end groups (in the expected 1:1 ratio to the n-Pr end group), and a quite strong hydrogen response in terms of molecular weight reduction, with i-PP from the most regioselective system (rac-Me2Si(2-Me-Benz[e]ind)2ZrCl2) having the strongest decrease (6-fold) of P h n. This strong molecular weight reduction, together with the reduction in regioerrors, is consistent with chain release to hydrogen at secondary growing chains being much faster than at primary chains for these two zirconocenes, since no isobutyl end groups were detectable. An appreciable amount of internal regioerrors can still be observed in the i-PP samples prepared in the presence of H2 with both systems, showing that, at the low amount of hydrogen used, propene insertion into a secondary growing chain is still competitive with hydrogenolysis. The authors realized that catalyst activation is too low to account for the increase in n-butyl end groups and postulated two possibilities, that is, either the inequality of the rate constants of hydrogenolysis for secondary and primary growing chains or a reinitiation slower than propagation. A scenario considering a reaction similar to the latter hypothesis has been modeled and is discussed further on. Carvill and co-workers519 carried out a similar study on a broader set of zirconocenes, confirming the results of Mitsui and Kashiwa, and Ju¨ngling and coworkers, and reached the same conclusion that the


extent of catalyst activation is not fully consistent with the amount of n-butyl end groups. Also the results of Lin and Waymouth (Table 19)372 hint at the inadequacy of the secondary chain activation mechanism. The second mechanism requires the formation of a zirconocene allyl complex, which is in turn formed from the Cp′2ZrH(CH2dCMeP), the product of β-H transfer after a primary insertion (see section V.C). This mechanism is more general, since it can work with any polymerization catalysts. Here, hydrogen activates the catalyst by converting the Mt(allyl) species, which have been shown to be effectively deactivated with respect to propene insertion,144,225,229 into Mt-H, that is, by shifting to the left the equilibria

[Cp′2ZrH]+ + CH2dCMeP h [Cp′2ZrH(CH2dCMeP)]+ h [Cp′2Zr(CH2-CP-CH2)]+ + H2 Since in all cases H2 generates Mt-H as the initiating species, the issue of propene insertion into the Mt-H bond deserves consideration. Some research groups have detected, under specific polymerization conditions and with zirconocenes of quite different regioselectivities, the presence of the 2,3dimethylbutyl end group,528-531 which must arise from a secondary propene insertion into the Zr-H bond (Scheme 48). Moscardi has modeled the insertion of propene into the Zr-H bond and found that secondary propene insertion into the Zr-H bond with formation of the Zr(i-Pr) initiating species is indeed competitive with primary insertion, even on highly regioselective catalysts.529 This molecular modeling study has also shown that the Zr(i-Pr) initiating species is slower compared to Zr(n-Pr) with respect to the following propene insertion and that isomerization of Zr(i-Pr) to Zr(n-Pr) can follow a relatively low energy pathway by associative displacement with the monomer after β-H transfer on the Zr(i-Pr) species (Scheme 49). This scenario can explain, at least in part, the lower than expected catalyst activation by low hydrogen levels observed by some authors: hydrogenolysis reactivates both Zr-secondary growing chain and Zr(allyl) species with formation of Zr-H initiating species, but at the same time the activation is limited by the formation of secondary, slower Zr(iPr) initiating species, which require either isomerization to n-propyl or a new hydrogenolysis to be converted in the faster centers. This mechanism explains why, for example, the 2,3-dimethylbutyl group is not observed at low propene concentrations (isomerization is faster than primary insertion into the Zr(i-Pr) species)252,519 or high hydrogen concentrations (hydrogenolysis is faster than primary insertion into the Zr(i-Pr) species),529 and also explains the lower than expected activating effect of hydrogen on systems which are not fully regioselective467 and the activation of highly regioselective systems such as the aspecific (Me5Cp)2ZrMe+225


Resconi et al.

Table 19. Influence of Hydrogen in Propene Polymerization with Bis(2-arylindenyl)zirconocene/MAO Catalysts372 catalysta

hydrogen, mmol

activity, kgPP/(mmolZr h)

relative activity

M hn

% mmmmb

% 2,1c

(2-Ph-Ind)2ZrCl2 (2-Ph-Ind)2ZrCl2 (2-Ph-Ind)2ZrCl2 [2-(3,5-(CF3)2-Ph)-Ind]2ZrCl2 [2-(3,5-(CF3)2-Ph)-Ind]2ZrCl2 [2-(3,5-(CF3)2-Ph)-Ind]2ZrCl2

0 2.6 3.9 0 2.6 3.9

18 39 111 3.7 13.0 42

1 2.2 6.1 1 3.5 11

130 000 12 000 12 000 68 000 8 000 6 000

51 28 25 70 64 56

trace 0.1 (0.1) nd (0.1) 0.2 (0.2) nd (0.2) nd (0.3)

a Polymerization conditions: bulk, T ) 20 °C, Al 13C NMR. b The values are referred to the total methyl p MAO/Zr ) 3000, From signals. c Total of internal regioerrors (total including end groups). nd ) not detected (see section VII).

Scheme 49a

a

Reprinted from ref 529. Copyright 1999 American Chemical Society.

and the isospecific rac-Me2C(3-t-Bu-1-Ind)2ZrCl2/ MAO.230,529 However, the possibility that hydrogen reactivates other types of reversibly deactivated metal species (such as Mt-Mt dimers, Mt-Al complexes), should be always kept in mind. One might consider that all these mechanisms will operate to different extents, depending on the type of catalyst and the polymerization conditions. Analysis of the unsaturated region of the 1H NMR spectra of i-PP prepared with different chiral zirconocenes in the presence of hydrogen shows that H2 addition suppresses the formation of internal vinylidenes,458,519 lending further support to the hypothesis of the allyl intermediate. We have also observed that the highly isospecific, but poorly regioselective, rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2/MAO produces i-PP with roughly the same amount of 2,1 units with or without hydrogen, despite the fact that catalyst activity is increased by H2 addition, and that ethene is inserted after a primary or a secondary unit without preference, showing that, for this catalyst system, a secondary growing chain is not a dormant site. Apparently, also isotacticity is affected by hydrogen, and opposite results have been reported for different systems. Tsutsui and Kashiwa305 reported a slight decrease in stereoregularity, from 91.7 to 89.0% mm triads. A stronger, negative effect was found by Lin and Waymouth (Table 19).372 No effect of hydrogen on isotacticity has been found in liquid propene at 50 °C with the rac-Me2C(3-t-Bu-1-Ind)2ZrCl2/MAO catalyst system.529 Also Carvill reported that low hydrogen levels do not influence tacticity.519 Opposite results have been reported for rac-C2H4(1Ind)2ZrCl2/MAO and rac-Me2Si(1-Ind)2ZrCl2/MAO un-

Figure 52. PP melting point as a function of isotactic pentad content. Data from our laboratories and from ref 295.

der very similar conditions (benzene, [M] ∼ 1.2 mol/ L, Tp ) 30, 60, 80 °C) to those employed by Kashiwa, but with a higher H2 concentration.252 This aspect of the influence of hydrogen clearly requires further investigation.

X. Outlook Metallocene catalysts are emerging as a successful catalyst technology and, to our view, represent the future of polyolefins not only in the realm of specialty polymers but, in the long run, for improved performance commodities as well. However, in the case of highly crystalline i-PP, despite a general belief to the contrary, metallocene catalysts are far less active and efficient than the newest supported Ti/MgCl2 catalysts and are unlike to replace them in any foreseeable future. So, why use metallocenes to make i-PP? In one sentence, because i-PP properties can be tailored! For example, i-PP can be made from fully amorphous to highly crystalline and anything in between. This full control over the degree of isotacticity means control over crystallinity and melting point (Figure 52), which recently allowed the preparation of thermoplastic elastomers of varying degrees of crystallinity. Soon, some of these new PP grades will target applications typical of polystyrene and plasticized PVC. In general, it seems likely that the success of metallocene catalysts for propene-based polymers, if any, will come from the production of materials that cannot be made with heterogeneous catalysts. In terms of catalysis, there are several issues that still require investigation. One is, obviously, improving ligand design and synthesis to lower manufacturing costs, on one side, and improve molecular weight


control, on the other. Still, some mechanistic points need to be clarified, such as growing-chain-end isomerization reactions and their dependence on catalyst structure and polymerization conditions. Understanding the details of the mechanisms of regio- and stereoselectivity is the key to rational catalyst design. To achieve this goal, a full understanding of the mechanisms driving enantioface selectivity and chain growth/chain release processes is required. The details of catalyst-cocatalyst interactions, although well studied in model systems, are far from understood on the real catalysts, for example, on supported systems. Last but not least, catalyst activity will have to be improved: apparently, we have not yet reached the point of monomer diffusion limitations, even with the most active systems. To close on a positive note, we expect several more years of productive research in metallocenePP.

XI. Acknowledgments The development of metallocenes is the story of the successful collaboration between organic, organometallic, inorganic, theoretical, polymer, and material chemists (and patent attorneys and research managers, too!), and as for many other technological revolutions, has been based on teamwork, stimulating competition, and free scientific exchanges. It has been both the scientific challenge and getting to know the human being behind the scientists that have made these 15 years of research so exciting for us. We are deeply indebted to all our colleagues at the Montell Research Centers in Ferrara and Elkton, at the Shell Research Center in Amsterdam, and at the University of Naples and Salerno, and to John Bercaw, Hans-Herbert Brintzinger, Vincenzo Busico, Paolo Corradini, John Ewen, Umberto Giannini, Robert Grubbs, Gaetano Guerra, Richard Jordan, Ilya Nifant’ev, Robert Waymouth, Adolfo Zambelli, Tom Ziegler, and many other scientists, who, through both their published work and many personal discussions, provided us with the present state of understanding of these highly sophisticated catalyst systems. We also thank Isabella Camurati for the NMR analysis of the polypropenes discussed in this review, Pasquale Longo for the sample of chain-end-controlled i-PP, and Steve Miller and John Bercaw for a sample of s-PP. L.C. thanks the MURST of Italy for financial support (Grant PRIN98, “Polimerizzazione stereoselettiva: nuovi catalizzatori e nuovi materiali polimerici”).

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